show track duration
fix assertion
handle audio 'ended' event to play next track

this is a bit more complex than I thought it would be
1) the react gubbins for events doesn't include media events
so we can't just do onEnded in the component attributes.  Have to put it
in IDidMount instead
2) audio events don't bubble, so setting a listener on a containing
element doesn't help
server/tracks-data becomes its own fn

easier to experiment if we can see the data structure before sending it
to the json mincing machine
sort results by artist/albukm/track0num
Merge branch 'master' of github.com:telent/sledge