Add keyboard navigation of posts

Closes #3015, ref #3469 
- Added stepThroughPosts method to PostsRoute, takes an integer, steps 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)

@rwjblue Just realized what this is. This is from when I scratched up some code for "Hey, this would be a *bad* way to do it, right?", ha.