CONFBRIDGE_INFO function to get conference data

Added the CONFBRIDGE_INFO dialplan function to get information about a
conference bridge including locked status and number of parties, admins, and
marked users.

Review: https://reviewboard.asterisk.org/r/1271/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@323517 65c4cc65-6c06-0410-ace0-fbb531ad65f3