Check for NULLs from image read functions.

Fixes segfaults on older ImageMagicks.
Improve color somewhat.
First crack at porting to cffi: load the library!
Rough port of parts of sanpera.image to cffi.
Most of the rest of sanpera.image; bunches of headers.
Make the basic `convert` tests work.
+pixel.h
Reworked exceptions; pixel views; colors.
Ported core.  Rather, added it to the end of _api.
Port geometry.
Port format.
Port filters.
Add a few more bits; more tests passing!
Remove all the Cython cruft.
Add bdist support, I think, probably.
Remove Cython compat shim.
Merge branch 'cffi'