res_pjsip_exten_state: Presence for digium phones

Added presence support for digium phones.

Review: https://reviewboard.asterisk.org/r/3239/
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Merged revisions 408882 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408883 65c4cc65-6c06-0410-ace0-fbb531ad65f3