Hopefully initial working implementation of the voting system

Assigned a track's uri to its upvote/downvote buttons so that they can
go to the Parse DB and increment/decrement the track's rating
respecively.
Assigned empty initial "data-url" values to track's iframes and upvote/downvote buttons

HTML5 allows giving tags custom attributes starting with the "data-"
prefix. We're taking advantage of this by giving voting buttons direct
knowledge of which track they will modify the rating of.