Fix bug: button showed "Play" while still playing

STR:
1. Be playing the last song in the playlist
2. Click on "Next Track"

Expected behavior:
Last song continues playing, and "Pause" button continues to be
captioned "Pause".

Actual (until this commit):
"Pause" button changed caption to "Play" even though song was still
playing.
Use this.$ in Backbone views

Inside of a Backbone view, these two patterns behave identically:

    $('some selector', this.$el)
    this.$('some selector')

This commit changes the first pattern to the second, terser and more
readable one.