first commit to achieve PyPy/cffi compatibility
use setuptools to download depencencies for pypy
accept bytes-like only on valid_compress function
Always installl cffi backend as fallback.

Merge py2 and py3 tests.
Remove py3k tests.
add travis and requirements file
add cffi backend to achieve pypy compatibility
add cffi as a dependency to use with pypy. Add test in readme for cffi