[WIP] Loading substates

Proposal/Discussion:
http://discuss.emberjs.com/t/proposal-nested-loading-routes/2895

Demo:
http://jsbin.com/ucanam/1399#/foo/bar/woot

Possible doubts:
- This change exposes `parentRoute` on `Ember.Route`. This could
  potentially cause issues if people have apps that reuse
  Route instances between two different route hierarchies,
  but this is happening.
- `Route#send` has changed to fire on that route and bubble
  to its parent, vs. just forwarding to `Router#send`. This
  means that you can fire events on routes that aren't currently
  active, which is a desirable feature
  (see https://github.com/emberjs/ember.js/issues/2943#issuecomment-20715840),
  but isn't strictly semver depending on whether you consider it a
  bugfix. My hunch is that most people won't be affected,
  but hard to say. @lukemelia has opinions.
- Global `LoadingRoute` behavior has changed (though it's decently
  agreed upon that it was always mad broke). Instead of creating a
  top-level sibling view of `ApplicationView`, (sub) loading routes
  render into an outlet (unless you override `renderTemplate` to
  do otherwise). This will surprise avid users of `LoadingRoute`,
  wherever they are. There doesn't seem to be a clean way to preserve
  this behavior in a backwards compatible manner without putting
  mental/API hurdles in front of doing the likely more desirable
  new behavior of rendering loading templates into outlets,
  but I'm all ears.

TODO:

- [ ] Extend this logic to `error` routes/templates
- [ ] Make `loading` action support `_super`
- [ ] Come up with more JSBin examples for everyone to grok.