synchronize omxplayer with expect

temporary solution to #2.  omxplayer prints this right when it starts
playing a song:

Audio codec mp3 channels 2 samplerate 44100 bitspersample 16
Subtitle count: 0, state: off, index: 1, delay: 0

Using pexpect to wait to see “Audio” in the mp3 thread and send a start
signal to other threads