Allow setting easing params

[chatziko]

An unfortunate side-effect of 37a09ae48 is that it made impossible to set easing params for the classes that support them (Elastic and Back). This is because:

• sub-types (ElasticEaseIn, ...) are now private so they can't be used directly

• Before one could do:

  var ease = Elastic.easeIn;
  ease.a = 0.2;

  but now this will mess all tweens up!

Not sure what's the most elegant way of fixing this, but here's an attempt:

1. I added easeInWith / easeInOutWith / easeOutWith functions for passing params, eg:

   Actuate.tween (MySprite, 1, { alpha: 1 }).ease (Elastic.easeOutWith(0.3, 0.5));

2. I set the properties themselves to (default, null) to avoid surprises.