res_stasis: Enable transfers and provide events when they occur.

res_stasis: Enable transfers and provide events when they occur.

This change enables transfers within ARI created bridges and adds events
for when they occur. Unlike other events these will be received if *any*
subscribed object is involved in the transfer.

(closes issue ASTERISK-22984)
Reported by: David M. Lee

Review: https://reviewboard.asterisk.org/r/3120/
........

Merged revisions 407153 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@407154 65c4cc65-6c06-0410-ace0-fbb531ad65f3