first pass at lastfm authing and scrobbling

This introduces Last.FM scrobbling by allowing a user to authenticate via the `rockstar` gem and scrobble videos that are listening to in full. This also updates the users "now playing" property that shows up on the user's Last.FM profile.

To auth a user goes to their settings page and clicks the "Grant Last.fm Access" button... after that the user is done.

If anyone's paying attention to the repo, any code review would be much appreciated. Mentioning a few who might be interested @macobo @betawaffle @TheABrad :cake: