Convert tabs to use Ember state machine

[agrieser]

Performance improvement, don't render things that aren't being shown

[tonywok]

I have a working version of this. I decided it was better off in a separate branch until I can get some code review having never worked with ember state managers.

The branch lives here: https://github.com/edgecase/spotbox/tree/tab_state_machine

known issues:

• Couldn't get ember to render the view of the initial state (dirty hack to make it on page load)
• PlaylistTab has a subview that would probably be better off as a CollectionView (playlist item)
• Quite a bit of repetition in the tab_bar view

[tonywok]

I was able to get rid of the hack by creating the state manager without an initial state, then doing a goToState inside of the Spotbox.ready function.

I decided not to make a CollectionView of playlist items. I wouldn't mind if someone took a look at cleaning up the playlist tab though.

This has been merged into master.