External MWI AMI support.

The external MWI AMI interface provides a thin wrapper around the core
external MWI resource.

The resource adds the following AMI actions:
MWIGet,
MWIDelete, and
MWIUpdate.

(closes issue AFS-46)

Review: https://reviewboard.asterisk.org/r/3061/
........

Merged revisions 404954 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404955 65c4cc65-6c06-0410-ace0-fbb531ad65f3