"Regular" Spritesheet support.

I'm about to start using StageXL with some old timey spritesheets -- the kind you find http://opengameart.org/ for instance.

Are you willing to accept a patch to `BitmapData.dart` similar to how texture atlases are supported via `BitmapData.fromTextureAtlasFrame`? Something like `BitmapData.fromSpriteMap`?

(last time I did any game programming it was all about the BitBlt -- sorry if I get any terminology wrong)