Port and update docs for Image, ImageChops, ImageColor, and ImageDraw

I might end up doing more this weekend, but these are done. [Preview is here again.](http://irskep-pillow.readthedocs.org)

Specific updates:
* Port PIL Handbook sections for the modules mentioned above
* Update the docstrings for those modules (except `ImageDraw`, presented Sphinx-related difficulties solved by writing the function signatures in the rst files instead of docstrings).
* Slight tweaks to the table of contents