Add .animation-*() mixins

[zlatanvasovic]

I fulfilled the promise.

[mdo]

Let's drop the animation- prefix on all the parameters please—makes it easier to write out if you need to specify for one reason or another. Plus just cleans the whole thing up.

[zlatanvasovic]

Done.

[cvrebert]

This needs documentation.

[zlatanvasovic]

@cvrebert It's done.