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 :)