New SIP Channel Driver - Add CLI/AMI initiated NOTIFY requests

Added the ability to send unsolicited NOTIFY requests to a particular endpoint
with a configured payload.  Added both CLI and AMI support.  For a given
endpoint, this module will iterate over all its contacts sending the appropriate
NOTIFY request to each.

(closes issue ASTERISK-21436)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2623/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393364 65c4cc65-6c06-0410-ace0-fbb531ad65f3