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: http://svn.asterisk.org/svn/asterisk/trunk@391297 f38db490-d61c-443f-a65b-d21fe96a405b