Multicast IP support and automatic audio device selection

I'm experimenting with using OpenOB to relay a TX feed to multiple OpenOB clients inside our University network, with as close to exact synchronisation between all the clients.  Hopefully this should be possible by adding the following multicast parameters to the transmitter and receiver.  I haven't had a chance to test this extensively yet but it worked inside our office network streaming from a Debian server to a Pi and a desktop machine running an Ubuntu VM with near-perfect synchronisation - I assume the slight discrepancy was due to latency inside ALSA/Pulse on either the Pi, the Ubuntu VM, or most likely both.

The other commit simply adds a default option to automatically choose the audio output to whatever GStreamer thinks is best.  I actually added this to try using the software in Windows 7 - it does work but the audio can be choppy at times, which is probably due to GStreamer being decidedly dodgy in Windows.  Also, the last OSSbuild for Gst-0.10 was before Opus was added, so you must use PCM or Celt.  Anyway, adding automatic selection might help people who are looking to set the software up simply on systems that use PulseAudio or possibly MacOS, if the rest of the program works there!