Stasis - Refactor AOC Events

Refactored the AMI events in AOC onto Stasis-Core.  The ast_aoc_manager_event
function now publishes a channel snapshot, along with a JSON blob describing
the advice of charge.  A "to_ami" handler has also been added that converts
the channel snapshot and AOC event data back into the appropriate data structure
for use with AMI.

(closes issue ASTERISK-21472)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2643/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@393449 f38db490-d61c-443f-a65b-d21fe96a405b