cffi backend implementation - pypy compat.

Hello!

This is a work to make snappy compatible with pypy using CFFI. The new framing sutff is not working yet but this won't be difficult to achieve in near future.

It was tested unser py27, py33 and PyPY.

What about moving sources to `snappy` folder and creating `snappy/tests` (or `tests` in the root)?

regards,