Merge changes from team/russell/issue_5841:

This patch adds a "Bridge" Manager action, as well as a "Bridge" dialplan
application.  The manager action will allow you to steal two active channels
in the system and bridge them together.  Then, the one that did not hang up
will continue in the dialplan.  Using the application will bridge the calling
channel to an arbitrary channel in the system.  Whichever channel does not
hang up here will continue in the dialplan, as well.

This patch has been touched by a bunch of people over the course of a couple
years.  Please forgive me if I have missed your name in the history of things.
The most recent patch came from issue #5841, but there is also a reference to
an earlier version of this patch from issue #4297.  The people involved in writing
and/or reviewing the code include at least: twisted, mflorrel, heath1444, davetroy, 
tim_ringenbach, moy, tmancill, serge-v, and me.  There are also positive test
reports from many people.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@61281 65c4cc65-6c06-0410-ace0-fbb531ad65f3