Added center_to and scale_to animations.
Fixed scale_to and center_to.

Tweaked scale_to to work both with an array/CGSize and a number.
Added tests for all 3: center_to, scale_to(x), scale_to(x,y).
Updated README and gitignore.
Merge pull request #125 from rolandleth/master

Added center_to and scale_to animations.