Add support to easing animations

[ruyadorno]

That'd be really hot

• http://www.robertpenner.com/easing/
• https://github.com/danro/jquery-easing/blob/master/jquery.easing.js

[ruyadorno]

We should recommend this script for people wanting to use more easing functions: https://github.com/jimjeffers/Easie/blob/master/easie.js