Async Support for Postal?

@drewr, what do you think about using core.async to make postal async? I'm imagining that send-mail would return a channel, rather than blocking.

I started working on this, thinking that `JavaMail` was actually async (based on the existence of the `TransportListener`; looks like that's not the case, unless I'm missing some obvious way to have `transport.send` return immediately.

This pull request starts the feature off. This is mostly to start discussion.