[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
Merge pull request #4760 from machty/ef4-transition-hooks

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