Add receive audio support

Requires [add_decode patch](https://github.com/perrym5/opus-ruby/pull/1) for opus-ruby.

**Major bug**: does not properly merge audio from multiple sources. Only one member of the channel can be "talking" at a time to hear clean audio.

Also note: The output fifo must exist prior to calling `client.receive_raw_audio( filename )` (or else it'll start writing to a regular file). There is a way for ruby to create a fifo, but it requires a dependency (and I don't feel it is my call to add another dependency).