CFFI bindings for usage with PyPy

[thedrow]

Currently the Python implementation uses the C API which is very slow on PyPy.
A CFFI implementation can be portable between CPython and PyPy and it's also easier to maintain for Python developers.
There is a pure python implementation of ketama named hash_ring but I believe that ketama will still provide a performance boost for PyPy.
What do you think?