Exposing `FlxTween.backward`
Making `backward` read-only.
Merge branch 'dev' of https://github.com/HaxeFlixel/flixel into dev

Conflicts:
	flixel/tweens/FlxTween.hx
Removing unnecessary backward set
Fixing minor issues with `FlxParticle.lifespan` behavior.
Making `FlxTween.backward` public and read-only.
Merge branch 'dev' of https://github.com/SeiferTim/HaxeFlixel into dev

Conflicts:
	flixel/tweens/FlxTween.hx
Merge branch 'dev' of https://github.com/SeiferTim/HaxeFlixel into dev
Tiny code cleanup