Add outbound resource helper.

We need a helper for the outbound resource. Things I'd specifically like to do include:

* Replace ``config.sms_tag`` with ``config.endpoints`` which is just a list of valid endpoint strings.
* Have a ``.send_to(opts)`` method with opts that include ``content`` (required), ``endpoint`` (required and must be in config) and ``contact`` (defaults to current user).
* Maybe ``to_addr`` (also defaults to current user and mutually exclusive with ``contact``).

Maybe config needs to somehow include which address contact field to use when sending?