Rewrite TextureAtlas system to support Json files

Using the Json Serializer classes, TextureAtlas has been rewritten to support simple Json formatted files.  It's still capable of loading legacy atlas files.  This is done to prevent code duplication of parsing and writing between the loader and the TexturePacker classes.  Additionally, this is done to create a cleaner format that is not reliant on needing an entire specialized parser for something used by only one aspect of a system.