ARI - GET /ari/asterisk/info

ARI - GET /ari/asterisk/info

This patch adds basic system information access to ARI.

The results are roughly what you get from 'core show settings', with a
few minor differences.

 * Data is structured, with 'build', 'system', 'config' and 'status'
   sub-objects.
 * Each sub-object is selectable, using the ?only= parameter. A comma
   separated list can be provided to select multiple sections.
 * A few config options are numeric, for which 0 means 'unlimited'.
   Instead of having a special interpretation of those fields, they
   are simply omitted if they're 0.
 * The information is limited to what might be useful to building
   external applications.

(closes issue ASTERISK-21575)
Review: https://reviewboard.asterisk.org/r/2702/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396125 65c4cc65-6c06-0410-ace0-fbb531ad65f3