ARI: Add mailboxes resource for controlling and polling external MWI

Adds the following AMI commands:
PUT mailboxes/mailboxName
    modifies mailbox state and implicitly creates new mailboxes
GET mailboxes/mailboxName
    retrieves a JSON representation of a single mailbox if it exists
GET mailboxes
    retrieves a JSON array of all mailboxes
DELETE mailbox/mailboxName
    deletes a mailbox
Note that res_mwi_external must be loaded for these functions to
actually do anything.

Review: https://reviewboard.asterisk.org/r/3117/
........

Merged revisions 405553 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405554 65c4cc65-6c06-0410-ace0-fbb531ad65f3