res_pjsip_notify: add dialplan application

Add dialplan application PJSIPNOTIFY to send either pre-configured
NOTIFY messages from pjsip_notify.conf or with headers defined in
dialplan.

Also adds the ability to send pre-configured NOTIFY commands to a
channel via the CLI.

Resolves: #799

UserNote: A new dialplan application PJSIPNotify is now available
which can send SIP NOTIFY requests from the dialplan.

The pjsip send notify CLI command has also been enhanced to allow
sending NOTIFY messages to a specific channel. Syntax:

pjsip send notify <option> channel <channel>