Add MASTER_CHANNEL() dialplan function, as well as a useful usage.

(closes issue #13140)
 Reported by: cpina
 Patches: 
       20090807__issue13140.diff.txt uploaded by tilghman (license 14)
 Tested by: lmadsen
 Change-type: feature


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@215301 65c4cc65-6c06-0410-ace0-fbb531ad65f3