Add a test event that reports changes in ConfBridge state

Add a test event that reports changes in ConfBridge state

This patch adds a test event to ConfBridge that reports transitions between
states in ConfBridge.  This is used by tests in the Asterisk Test Suite
that verify state changes based on the entering/leaving of conference
participants.
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Merged revisions 376414 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 376415 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@376416 65c4cc65-6c06-0410-ace0-fbb531ad65f3