FR: add a dialMusic equivalent to SIP-REFER

[javibookline]

From the user's perspective, a SIP-REFER is seen exactly as a Dial, and we would like to give feedback to the user that their call is being transferred by playing a beep sound from a file.

This is currently possible in Dial verbs but not if the call is transferred via SIP-REFER.

[davehorton]

With a SIP REFER we are out of the media flow fairly quickly. Its not clear that we can play anything that the user will hear. To test this, you might use the mod_dub feature in a config verb to be playing some music -- start it before the REFER, do the REFER and see how quickly the music gets shut off. If you are able to hear the music until the remote party answers, provide us with a SIP trace of the call.