* 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
Added the remaining features of the .TMX Tiled Map format. Removed the parts of the format which aren't needed for rendering, like `TiledMapTerrainContent`. Terrains and the newer WangSets are only used in constructing a map, not for rendering. Added exception throws for non-implemented parts of the format.
`FirstGlobalIdentifier` is only used with the map to reference the tileset which contains the tile referenced. It isn't really part of the definition of a `TiledMapTileset` even though the `.TMX` format includes it with the `<tileset>` tag
Dylan Wilson