confbridge: Add Duration to ConfbridgeList event

The ConfbridgeList event doesn't include how long the user has been a
member of the conference.  This patch adds Duration (seconds) which
is based on user->chan->answertime.

Tested by: George Joseph
Review: https://reviewboard.asterisk.org/r/3955/
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Merged revisions 422444 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 422445 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422446 65c4cc65-6c06-0410-ace0-fbb531ad65f3