Implement TiledImageLayer

Hello!

I'm making a new game with some people and while doing so we've added some new features to pytmx. We've added TiledImageLayer to handle tiled Image layers, included a new list with all the tiled layers in proper render order and changed the data array size from unsigned char to unsigned short (this was problematic with some maps where hundreds of different tiles are used).

I hope it's useful. Please tell me if there is anything I have to do before merge.