rtpbasepayload: add pt and ssrc to stats
rtspconnection: Fix connection_poll()

* Only check for conditions we are interested in.
* Makes no sense to specify G_IO_ERR and G_IO_HUP in condition, they
  will always be reported if they are true.
* Do not create timed source if timeout is NULL.
* Correctly wait for sources to be dispatched, context_iteration() is
  not guaranteed to always block even if set to do so.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726641
rtspconnection: Fix minor memory leaks in error handling

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726642
mikey: add MIKEY parsing helpers

MIKEY is defined in RFC 3830 and is used to exchange SRTP encryption
parameters between a sender and a receiver in a secure way.
This library implements a subset of the features, enough to implement
RFC 4567, using MIKEY in SDP and RTSP.
docs: add MIKEY docs
rtsptransport: UDP is also default for SAVP and AVPF
mikey: fix return values of g_return_*
rtpbasepayload: Implement video SDP attributes

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726472