Support for TMX Image layers

[nelsonxb]

The TMX format supports layers consisting of an image, which apparently Cocos doesn't support yet. This is useful for specifying backdrops (for example).

Ideally, this would include support for:

• Loading and rendering the image (obviously)
• Offsets
• Transparent colour
• Opacity

[nelsonxb]

I plan on implementing this regardless of all else, since I have a use case that requires it right away. Unless there are any objections, I'll also refine it and submit a PR.

[ccanepa]

good, would merge