[WIP] Initial implementation of async resolver

[tailhook]

This is a very dumb implementation, i.e. it doesn't implement timeouts. But it works in simple cases.

It can't be merged yet because it relies on tokio-rs/tokio-core#35 being merged in, but you can get the basic idea. So I'm open to suggestions.

[sbstp]

Any updates on this? Is it ready for merging?

[tailhook]

No updates yet. I'm not sure if I'll find some time to update.

Basically, I'm experimenting with a domain library which was published since this pull request.