Implement FlxG.assets

Flixel has no assets manager to dynamically load images/sounds on demang, so a lot of projects just embed them at compile time. It leads to an excessive use of memory that could be better managed.

I would be great to have an asset manager available at `FlxG.assets`. We don't have to write our own from scratch, we can port an existing one from another game engine (e.g. Starling).
