This is fairly much a straight copy of the Opus payloader from
gst-plugins-good, with two trivial modifications that weren't
acceptable upstream:

 • Change RTP clock rate to 16000 (#793765)
 • Force mono encoding because the 'stereo' property doesn't work (#793763)

I think the latter is a bug which should be fixed upstream, but
the former is undesirable there because 16000 is not a standard
clock rate and we don't really want people to use it by accident.

So we end up with our own copy, and thus it's easier just to use
the brute force answer to the second bug because we never want stereo
in our version anyway.

The licence header has been updated to LGPLv2.1 for uniformity
with the rest of the purple-chime code base.
