Add support for embedded tilesets

[TheBoneJarmer]

Like the title says really. Currently only external tilesets are supported but I should add logic to support internal or embedded tilesets as well.

[TheBoneJarmer]

I know it has been a while but I finally started doing this. This will result in an kind of different structure here and there hence this feature will be part of TiledCS 4.0.0.