Change chan_skinny to use core transfer API.

Changes for both attended and blind transfers in chan_skinny to use the new transfer API instead of masquerade.

(closes issue ASTERISK-21526)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2557/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391297 65c4cc65-6c06-0410-ace0-fbb531ad65f3