Drop Python2 support, change configuration filename and some minor improvements

• [Backward incompatible] Drop Python2 support (remove six, unicode litteral,
  update package configuration, etc..);
• [Backward incompatible] Changed default configuration filename from
  settings to boussole. This was required to avoid clash with some other
  projects since "settings" is a too common word;
• Drop pathtools package dependency since it is an abandoned project;
• Update doc config, use livereload and with new RTD config file;
• Drop support for libsass-python <0.19.4.
• Rename requirements_freeze.txt to frozen.txt;
• Add a new script for development which help to automatically update the
  frozen.txt file;

For new configuration filename change you have two way to resolve it:

• Just change your configuration filename to the new one, like if you were
  using settings.json, you will rename it to boussole.json;
• Use option --config to explicitely use your own configuration filename;

Last Python2 support, compatibility for recent Click versions and improve
performance

This will be the last version with Python2 support. A next release will come
soon to remove its support and focus on Python3 only. This will may also drop
support for old Click and libsass versions.

Click requirement has been relaxed to only require for version greater or equal
to 5.1.0. This has been currently tested to 7.1.2 so it's surely safe
from 5.x to 7.x versions.

Watcher has been modified to avoid performing indexation on every event and
every file. It should give a few performance improvements and also avoid a bug
with editors making a transition writing file when saving modification (like
a foo.scss.part when writing on foo.scss) which may leaded to incorrect
errors.

Since libsass-python 0.19.4, a new minor feature has been added which add the
column position in some error messages. This was breaking tests but without any
impact in Boussole functioning.

For sanity, we added a minor check against libsass-python version in a test to
continue support for libsass-python from 0.18.x to 0.20.x versions. It adds a
new dependency to packaging package only in development requirements.

Finally the Tox configuration has been updated to perform tests against supported
libsass-python versions and flake coverage has been done on tests.

Fixed warning from libsass-python about 'custom_import_extensions'

Libsass has reverted its previous change from version 3.5.3 which ignored CSS files on default. This has required to add a new option CUSTOM_IMPORT_EXTENSIONS in Boussole 1.2.3 to enable CSS files
support.

Since CSS support is back again on default, we removed useless setting CUSTOM_IMPORT_EXTENSIONS and don't use anymore custom_import_extensions argument with libsass-python compiler.

This remove previous warning from libsass-python about custom_import_extensions deprecation.

Fixed PyYAML 'load()' deprecation warning

For a recent security issue, PyYAML has introduced a change to its load() method to be more safe.

We now use the full loader mode so it does not trigger a warning anymore.

Indented Sass syntax support

Boussole should now be able to manage projects writed with the old indented syntax (in files with extension *.sass).

There is only one issue which blocking Boussole to manage @import rules on multiple lines. Also, multiline comments are not supported. It will lead to false positives if there are @import rules inside a multiline comments, causing these rules to be taken as correct imports to check for.

Fixed packaging, close #34

'setup.py' has been forgotted from previous release and still contained information.

So it has been cleaned an 'setup.cfg' has been updated to include missing '[options.entry_points]' section.

We moved every package informations into 'setup.cfg' and now
'setup.py' is only an entrypoint for setuptools. tox and pytest
configurations has been moved also into 'setup.cfg'.

Makefile has been updated and python-venv has been dropped in profit
of virtualenv to ease development.

Introduce a new way to load settings file with a discovering
which either just load given an explicit file path or try to find
it from base directory and available settings backends. This is
related to issue #30.

This should not include backward incompatible behavior, it just adds
capacity to find another backend default filename kind.

Concretely, before this release only 'settings.json' would be finded
when no explicit file path was given and now it will be able to find
also a file 'settings.yml' if it exists in current directory.

Minor release just to update some tests requirements related to pytest

Minor release with some documentation update and validation for libsass=0.13.4

Add --poll option to watch command to be able to use the Watchdog polling observer insteand of the native platform observer.

Add YAML backend format for Configuration file

Upgrade 'libsass-python' dependancy to '>=0.11.2' to profit from 'libsass==3.3.6'

Added Python 3.4 support, thanks to @feth for its contributions.

Last beta version with some minor features:

• Added new settings to enabled sourcemap generation, #6;
• Finalize documentation, #10
• New CLI action to start a project, #8;
• Added logo to documentation;

First beta version with all important features and correct behaviors.

Boussole is almost near of Beta release with a better documentation, and working command line actions "compile" and "watch".

Added Finder to API