[FEATURE __] Loading/error substates

Proposal/Discussion:
http://discuss.emberjs.com/t/proposal-nested-loading-routes/2895

Possible doubts:
- 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:

- [x] Extend this logic to `error` routes/templates
- [x] Make `loading` action support `_super`
- [ ] Come up with more JSBin examples for everyone to grok.

## Demos

- [Basic Loading Substates Demo](http://jsbin.com/ucanam/1399#/foo/bar/woot)
- [Loading+Error Substates Demo](http://jsbin.com/ucanam/1406#/foo/bar/woot)
- [handling `error`, calling `_super`](http://jsbin.com/ucanam/1407/edit)