* Replace square in center of level with stationary ball
* Improve collision demo
- Collisions between balls are smoother.
- Fixed an issue with collision bounds not being set properly. (This is what made the collision appear unsmooth)
* Change from storing layers by index to Dictionary key
Models are now stored with the `TiledMapLayer` they are part of.
* Allow render calls via TiledMapLayer
* Add TiledMapGroupLayer and model build recursion for groups
* Add rendering support for TiledMapGroupLayer
* Update TiledMapReader to read TiledMapGroupLayers
* Update Features Demo to Include a Map with a Layer Group
* Update TiledMapWriter and TiledMapProcessor to Support Groups
* Update TiledMap to Report Layers Correctly
Dylan Wilson