Update TiledMapTileSet.java

Premise:
I am using Tiled Map Editor and giving properties, such as friction, to individual TileSet tiles as a template so that I may lay down 20 'ice' tiles and 20 'road' tiles without having to individually specify their respective properties (friction) for each tile laid as that would get cumbersome. 

TileSets support properties for their Tiles, and libgdx allows access to these TileSets, however the TileSet class only allows access to their respective Tiles (and their respective properties) if you know the exact id of said Tile.


Current State:
[*]You may iterate through all the TiledMapTileSets for a given TileMap.
[*]For each TileSet, if you [b]do [/b]know the TiledMapTile's id, you may get TiledMapTile's from the TileSet using TileSet.getId(id). 
[*]If you [b]do not [/b]know the id of the TiledMapTile, there is no way to iterate through all of its TiledMapTile's - such is my case since the user may make their own levels and the ids they may use are unknown to me
Update TiledMapTileSet.java

Made TileSet's Tiles iterable.
Merge pull request #362 from SG57/patch-1

Update TiledMapTileSet - get TileSet tiles when TiledMapTile ID is unknown