added support to be able to set the channel var TRANSFER_CONTEXT so when

a #transfer is executed it uses ${TRANSFER_CONTEXT} from transferree else
from transferer else it acts as always
-anthm


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@3499 65c4cc65-6c06-0410-ace0-fbb531ad65f3