Support for loading BitmapFonts from TextureAtlases

I haven't actually tested this yet, I'm in the process of doing that, but I wanted to get some feedback first.

Anyway, isn't it weird that `BitmapFont`s can be created from `TextureRegion`s, but you can't do so within a `BitmapFontLoader`?  Being able to do so would be really, really convenient.  Putting your fonts in the same atlas as your sprites would improve performance, and this PR makes it easier to do so.

An update to the `BitmapFontTest` is on the way.
We should just pass the string to the constructor, the loader for TextureAtlas may have a different resolved.