Add JSON API for Asterisk.

This provides a JSON API by pulling in and wrapping the Jansson JSON
library[1]. The Asterisk API basically mirrors the Jansson
functionality, with a few minor tweaks.

 * Some names have been asteriskified to protect the innocent.
 * Jansson provides both reference-stealing and reference-borrowing
   versions of several API's. The Asterisk API is exclusively
   reference-stealing for operations that put elements into arrays and
   objects.
 * No support for doubles, since we usually don't need that.
 * Coming along for the ride is the ast_test_validate macro, which made
   the unit tests much easier to write.

 [1]: http://www.digip.org/jansson/

(issue ASTERISK-20887)
(closes issue ASTERISK-20888)
Review: https://reviewboard.asterisk.org/r/2264/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378915 65c4cc65-6c06-0410-ace0-fbb531ad65f3