Add support for loading substates

See: https://github.com/emberjs/ember.js/pull/3568

- added the concept of an intermediate transition which happens immediately and doesn't cancel an existing active transition
- the "pivot handler" of a transition is provided on the Transition object
- added `inaccessibleByURL` as a handler property that will prevent URL transitions into this route, as well as prevent URL updates when `transitionTo`ing into this route. This ended up not being strictly necessary to the loading substate stuff, so maybe i should take it out?
- `getHandler` is now passed the name of the handler's parent route as a second param for those libs that don't plan on reusing the same handler instance between different routes. Looking at you, Ember.
- added `willResolveModel` event that fires when a destination route's `model` hooks are about to be called.