Add AMI_CLIENT dialplan function

Implementation of a dialplan function for checking manager accounts. Right now
it only returns the number of logged in sessions for a manager account, but
other attributes can be added later.

Patch by: Olle Johansson
Review: https://reviewboard.asterisk.org/r/421/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370943 65c4cc65-6c06-0410-ace0-fbb531ad65f3