Route inherits parent's model by default.

Opening up discussion to address [Core f2f proposal: unspecified this.route() models default to parent resource](http://discuss.emberjs.com/t/core-f2f-proposal-unspecified-this-route-models-default-to-parent-resource/3612/10).

Went with the simplest approach I could think of to make it work. All tests passing locally, including the one introduced.
Looking for c+c, thanks in advance :)

Possible improvements:
- [x] Test nested `route`
- [x] Test more than one level of nesting
- [x] features.json
- [x] FEATURES.md
- [x] prefix commit
- [x] only leaf route/resources inherit model