AMI hook interface

This patch, originally submitted by jozza, enables custom modules to send actions to AMI
and receive messages from AMI via a hook interface. Included is a simple test module to
illustrate the interface.

(closes issue #14635)
Reported by: jozza

Review: https://reviewboard.asterisk.org/r/412/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@227448 65c4cc65-6c06-0410-ace0-fbb531ad65f3