Cffi pixelaccess

Fixes #248, #67

This is a python /cffi implementation of the PixelAccess object and assorted hooks in Image.getpixel/putpixel. It is enabled by default for pypy, not for the c-python implementations.  It requires cffi to be installed at runtime, there are no build dependencies. 

This branch is (knock on wood) complete and consistent with the existing c-api implementation of pixel acces (assuming and including the i16-pixelaccess branch patches). It appears to be fast enough, but I haven't spent a ton of time tweaking it for ultimate speed.