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