Texture atlas

[jeremyfa]

I'd like ceramic to work better with texture atlases.

This is what is planned:

• [x] Add an Atlas asset type and support loading a few atlas file formats out of the box
• [ ] Add the possibility to create an atlas on the fly from images loaded with the same Assets object
• [x] Add the possibility to assign a rotated texture tile to any Quad to make the use of texture atlases much easier
• [x] Handle additional atlas region features (like offsets x/y) when using a more specific object (maybe Sprite object instead of Quad)