Feature transitions

See:
https://github.com/HaxeFlixel/flixel-addons/pull/97

This adds the ability to use transitions simply by calling FlxG.switchState(). It is using compiler conditionals so that there are no dependencies -- it automatically detects whether flixel_addons is in use.

(It also has a commit to fix the missing FillStyle typedef which was keeping flixel-addons from compiling)
Right, so FlxTransitionState might be poorly named. FlxTransitionState is a state *that can be transitioned from*, not the state that actually does the transition.

In any case, where do we go from here?