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. 
I've created a local branch with your changeset so far, so we can code things in parallel. You're welcome to merge it into your local repo: https://github.com/2600hz/kazoo/tree/lbueno-daitan-master

I think you still need to handle when someone calls into the box to check voicemails; will the external URL be accessed for fetching the recording?