Transitions

This adds a full-featured set of transitions to flixel.

Usage:

First, extend FlxTransitionState as ie, FooState
 
 Method 1: 

````  
  var in:TransitionData = new TransitionData(...);		//add your data where "..." is
  var out:TransitionData = new TransitionData(...);
  FlxG.switchState(new FooState(in,out));
````
 
 Method 2:

```` 
  FlxTransitionState.defaultTransIn = new TransitionData(...);
  FlxTransitionState.defaultTransOut = new TransitionData(...);
  FlxG.switchState(new FooState());
````

This also requires a small extension to flixel core itself, which I will make in a separate PR.
I think @Beeblerox wants to add support for playing animations backwards - would come in handy for this sort of thing. :)