Implemented action `FadeIn`, `FadeOut`, `FadeTo`
Merge branch 'master' of https://github.com/pistondevelopers/piston
Fixed doc
Unified the parameters of `ActionState`
Added `Ease(Rate)In`, `Ease(Rate)Out`, `Ease(Rate)InOut` actions

  Those Ease actions can be combined with interval actions like
  `MoveTo`, `RotateTo` to perform tweening animation.
Merge pull request #645 from Coeuvre/master

Implemented action `FadeIn`, `FadeOut`, `FadeTo`. Added `Ease(Rate)In`, `Ease(Rate)Out`, `Ease(Rate)InOut` actions