[WIP] loading/error substates

Proposal/Discussion:
http://discuss.emberjs.com/t/proposal-nested-loading-routes/2895

Demo:
http://jsbin.com/ucanam/1399#/foo/bar/woot

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.