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)
Sorry. I mean something along these lines:

(in transition setup)

```haxe
FlxG.signals.stateSwitched.add(function()
{
   if (transitionNeedsToPlay)
   {
      FlxG.switchState(transitionState);
   }
});
```