Add J and K to navigate between listed gems

Just as Google Reader and many other apps, use J to move the selection down, K to move the selection up and press enter to go to the page of the gem.

I would've liked to write unit tests for this, but there is nothing setup for this. Maybe time to use the asset pipeline and throw in something like Jasmine/evergreen?

PS. There is a lot of commented out code in `application.js`. What should we do with this?