Patch for decimal heights (due to Tiled editor) and ability to load external tilesets.

### Patch for decimal heights

NTiled failed to parse my TMX files due to there being decimals for my object heights. I am on the latest version of Tiled so I'm not sure when happened.

    <object name="EnemyEntity" x="2016" y="241" width="622" height="76.16"/>

### Loading external tilesets

We use TSX files to share tilesets between maps and it looks like:

    <tileset firstgid="201" source="meta.tsx"/>

I started by trying to load the xml within the TilesetParser, but it would need knowledge about where the original document was loaded from. We don't have this information and to avoid having to pollute the code I decided merging the xml documents would be a neater solution. If you would prefer another way let me know and I can take a look.