Tiles & both object types have the same pos & size if they're at the same place

Unfortunately, for non-tile objects, this means the y coordinate is
shifted by the height of the object from the value stored in the TMX file.
Mention that "staggered" is a valid map orientation

Since tmxlib doesn't currently do anything with the orientation,
no code changes are needed.
Allow loading image layers
Allow writing and JSON import/export for image layers
Add add_image_layer to Map, document the add_*_layer methods
Update CHANGELOG
Restore Python 3 compatibility
Add missing test files
Add support for ellipse objects
Update to pypng 0.0.14
Test on Python 3.3
Support polygon and polyline objects
Support map background color
Update CHANGELOG
Reorganize the project's file sctucture
Update documentation
Add readthedocs link to README
fileio: raise ValueError rather than "assert False"
Add support for terrains (XML only for now)
Terrain support for JSON/dict export