Modified TiledLevel.hx To support Multiple tilesets and collidable layers

Modified TiledLevel.hx to allow support for multiple tilesets per .tmx file. Changed a hardcoded '1' to the dynamic 'tileSet.firstGUID' value pulled from the tmx file.

Modified TiledLevel.hx to fix bug that only checked the first collidable
layer for collisions. Moved the 'FlxG.overlap' call to the condition of an if statement. If a collision occurs, it then returns true. If not, it continues the loop.