Change chan_unistim to use core transfer API.

Review: https://reviewboard.asterisk.org/r/2553

(closes issue ASTERISK-21527)
Reported by Matt Jordan



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@392436 65c4cc65-6c06-0410-ace0-fbb531ad65f3