Add keyboard navigation of posts

Closes #3015
- Added stepThroughPosts method to PostsRouter, takes a integer, goes that far, wraps around the array.
- PostsPostRoute notifies the PostsController of which model it currently has, to help stepThroughPosts know who's selected

![keyboardsc](https://cloud.githubusercontent.com/assets/1207005/3759349/e6c7abba-185b-11e4-9deb-c21e5c252f41.gif)