[FEATURE ember-routing-will-change-hooks] finer-grain transition hooks

Brought to you by @ef4, and demonstrated at his EmberConf talk:

http://confreaks.com/videos/3302-emberconf2014-animations-and-transitions-in-an-ember-app

See this router.js PR for details:

https://github.com/tildeio/router.js/pull/89

TL;DR adds `willLeave` and `willChangeModel` hooks (not that Ember renames the router.js `willChangeContext` hook to `willChangeModel`)