Subject: Transfer feature

The following capabilities have been added to the
transfer feature:

- The transfer initiation announcement prompt can
now be customized in features.conf.

- The TRANSFER_EXTEN variable now can be set on the
transferer's channel in order to allow the transfer
function to automatically attempt to go to the extension
contained in this variable, if it exists. The transfer
context behavior is not changed (TRANSFER_CONTEXT is used
if it exists; otherwise the default context is used).
