Add AppleScript variables to get playback progress.

This should close #157. Please note that “current song duration”
is different from “song’s duration” which does not exist. This is
because there is no clear easy way to get a song’s duration from
Pandora without playing the song. (Can look here to see what I
mean http://pan-do-ra-api.wikia.com/wiki/Json/5/station.getPlaylist)