Transitions

Even though I'm technically in charge of this repository I'm opening this pull request as a formality so I can coordinate with the other 3:

https://github.com/HaxeFlixel/flixel/pull/1274
https://github.com/HaxeFlixel/flixel-addons/pull/97
https://github.com/HaxeFlixel/flixel-demos/pull/146

And not forget what I'm doing, as when everything is ready to go, all 4 will have to happen simultaneously.