Allow non-pjax page updates.

Extract transition functionality from push.js into a new file, transitions.js.
Expose a global function, similar to PUSH named TRANSITION used to transition
from one page/view to another without dictating how the new markup is sourced.

This allows for Ratchet style page transitions when rendering client side.

I'll add comments to the diff shortly, highlighting some of the changes I've made.
@ndelage If transition is set, you end up calling `complete()` twice - once here and again when the transition completes.