[Asterisk-video] RTCP Support

[Andrew Buchanan]

Hi,

I've added some RTCP support in app_rtsp to send receiver reports back
to the server for servers that send sender reports.  I should clean it
up a bit and I'm not actually calculating any packet loss statistics,
but it's enough to keep some servers from timing out and dropping the
connection.

It involves a fair number of changes to app_rtsp though.  I would like
to also make a bunch of other changes to app_rtsp in the near future,
such as support for basic authentication, so I'll probably wait before
posting it unless somebody really wants it right away.  

I saw Sergio had a ticket for rtcp support 11 months ago, but
considering it's pretty boring and involves rfc's :-), I figured I'd
save somebody else the time.

Andrew Buchanan