Allow for adding message body to the SIP NOTIFY message

Ability has been added to both manager command SIPnotify as well as console
command sip notify. Message body is stored in the "Content" variable. An 
example is present in sip_notify.conf.

(closes issue #13926)
Reported by: jthurman
Patches:
      sip-notify-svn189463.diff uploaded by gareth (license 208)
Tested by: gareth


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@224035 65c4cc65-6c06-0410-ace0-fbb531ad65f3