Add Python bindings

This adds Python bindings (developed originally as https://github.com/khaledhosny/brotli-python). Everything is kept under `python` subdirectory, but distutils is very inflexible, so I had to symlink `dec` and `enc` directories from top level for the build to work (I wouldn’t insist on using a `setup.py` script myself, but it seems to be required for PyPI which I guess we would want to uploat to at some point). CC @behdad.