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.
The work of finding new bar elements was previously before transitioning new content as part of an old method `extractDom`. I don't think there's any reason to convert the markup of the new view into an object with keys like `bartab`, `barfooter` and `content`. So I'm running the selector here.