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)
Sounds good to me. I just looked at it and the signal won't be enough by itself as it's called halfway through the state destruction process. If you have an outline of what you have in mind I can go ahead and do that.