Rank Your Music game, using Spotify API

* Rank music according to Spotify's popularity index
* Temporarily authenticate using [Implicit Grant Flow](https://developer.spotify.com/web-api/authorization-guide/#implicit_grant_flow)
* Pulls a random playlist from your account (must have >1 song)
* Rearrange tracks using [Waffler](https://github.com/zenahirsch/jquery-waffler) jQuery plugin
* Check answers, red indicates the current track is less popular than the track directly below
* Preview song by clicking music icon in upper right-hand corner of each track
I'd move this callback to a separate named function, e.g. `onPlaylistsFetched()`.