gobject-introspection: Add annotations to support language bindings

In addition a few cosmetic changes:

 * Adjust the order of arguments
 * Fix typo: occured -> occurred
 * Fix indentation after Return:-clauses

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726941
tests: Improve code coverage for rtsp-session-media

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726940
tests: Improve code coverage of rtsp-threadpool tests

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726873
tests: Add unit tests for sessionpool

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726470
stream: add SRTP support

Install srtp encoder and decoder elements in rtpbin
Add MIKEY in SDP
test: print an error on failure
test: enable SRTP only on RTSPS

We only want to enable SRTP when doing rtsp over TLS so that we can
exchange the keys in a secure way.