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.
Fyi, this could be written as:

```haxe
anim = switch (Status)
{
   case IN: "in";
  // etc
}
```