kissfft

This is the first release with semantic versioning. It has the roughly the same code as v131, but some symbol visibility changes and build enhancements to improve shared object usage and use as a packaged/installed library.

• various improvements to the c++ version.
  ** use std::acos instead of acos in c++ version
  ** more standard use of scratchbuf in c++ version
• Travis CI was added
• SPDX license identifiers added for easy automated parsing
• minor changes related to switch from hg on sourceforge to git on github
• modernized some python tests