Implement FlxG.assets

[Dovyski]

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).