Added support for an external storage for voicemail medias.

I wanted to save the voice mail medias in a different place, like the custom URL that we have in Recordings callflow module. 
It was just to save in Couch an extra field to retrieve the media location later. In the  `store_recording/4` I use the `update_doc/4` function to update this field value. I've noticed that that function didn't work if the field value was not created before. Maybe a best solution would be evaluate the the URL before calling the `message_media_doc/3` in the `new_message/4` function.