chan_sip: Add SIPpeerstatus command to AMI

This patch was submitted by mnicholson a while back. It adds a new AMI action
which allows users to request SIP peer status on demand similar to existing
PeerStatus events and to the output you would see from CLI with sip show peer

Review: https://reviewboard.asterisk.org/r/1098/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370518 65c4cc65-6c06-0410-ace0-fbb531ad65f3