Move README badges to the beginning

Hooray for readme OCD, I guess
Split roundtrip tests into their own module
Use properties instead of mixins for "unpacked" properties
Better names for image tests
Coalesce image regions

Making a regon from an ImageRegion now makes a region of the ultimate
ancestor (hopefully an Image).

This also makes image regions more strict: they must always
fully overlap their parents.
Test the assert_item helper
Make images display nicely in IPython Notebook
Make image regions display nicely in IPython Notebook
Run image tests also with regions
Cleanly skip image comparation steps if PIL is not installed
Add drawable Canvas object, with the ability to paste images to it
Use slicing to obtain ImageRegions
Provide a DrawCommand interface for Canvas
Do image tests with a non-trivial region
Make it possible to render tile layers to draw commands, and images
Restore Python 3 compatibility
PyPNG should always be available, don't guard the import