Extend Tiled Map (CCTMXTiledMap)

I started working with Cocos2D-x and planning active using Tiled Map in my game. Some features are useful for gamedevs, I suppose.


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**Support "image layer" for Tiled Map**

* New tests in TileMapTest: TMXOrthoBackgroundOnlyTest, TMXOrthoBackgroundWithSpritesTest.

* New classes for CCTMXTiledMap: CCTMXImageLayer, CCTMXImageLayer, CCTMXImageLayerInfo.

* New method for CCTMXTiledMap: imageLayerNamed() - Return the TMXImageLayer for the specific layer.

* Correct error when tiled map (CCTMXTiledMap) not consists of tilesets.

* Correct potential error for parse of the image source of tilesets (added enum TMXPropertyTileSet).

* Assert for CCTMXLayer, when texture for tileset is not loaded.