More API Docs and functions!

I improved the API docs and also added the following functions to the tile rendering system:

• CLEAR.tiles.clearTileSystem()
  • clears the initialized tile system
• CLEAR.tiles.hideTile(tx, ty, value)
  • sets the hidden value for the tile at tx, ty to value (a boolean)
• CLEAR.tiles.isHidden(tx, ty)
  • returns the hidden value for the tile at tx, ty as a boolean
• CLEAR.tiles.removeTile(tx, ty)
  • removes the tile at tx, ty from the tile system

I also added a simple check to all the CLEAR.tiles functions to make sure the tilesystem is initialized before they run to prevent errors.