Merge pull request #2 from HaxeFlixel/dev

Updating to Match latest HaxeFlixel Version
Exposing `FlxTween.duration` to be able to change/check on duration on the fly.
Merge branch 'dev' of https://github.com/SeiferTim/HaxeFlixel into dev
Changed `target` into `duration` and made Public.
Merge pull request #843 from SeiferTim/dev

Exposing FlxTween.duration