Support cffi-0.6

Due to the enum format change from str to int in cffi-0.6, strings can't be passed as enum arguments.
This changes the constant generator to generate int constants and uses them everywhere instead of strings.
Back to a "stupid" generator for constants.py that doesn’t need CFFI.
Fix the constant generator for Python 3.
Simplify GDK enum constants access. See cffi pull request 11.
Move Matrix to its own module.
Docs: remove string constants that are now integers.
Changelog for 0.4, require CFFI >= 0.6
Merge branch 'cffi-0.6'

Conflicts:
	cairocffi/__init__.py
v0.4