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.
Also, how do you want me to report failure?  `Gdx.app.error`, throw an exception, `assert`, or...?