load_tmx() only generates a Tileset if the Tileset element contains an Image element

[benfrussell]

load_tmx() only generates a tileset if the tileset element contains an image element. TMX files can contain tilesets made up of tile elements, and if that's the case that tileset will not be loaded with cocos. This causes a crash later in the loading process if those tiles are used anywhere.

As of v0.9 of the tileset format, the image element for these tiles can be stored as a child of the tile.

My case of a valid tileset that will not be loaded correctly with load_tmx():

[benfrussell]

Created a pull request of a fix for this issue, #304