Provide DNS feature

[carllerche]

This is a placeholder issue for providing an official tokio-dns crate. I will try to write up thoughts soon.

Relates to #343.

[liranringel]

I think 'tokio-dns' is a problematic name since the process of resolving a host may not include dns at all (hosts file, nbns, etc...). Maybe 'tokio-resolve'?

And to be clear, that crate will use the naive os api, right?

[DemiMarie]

I think this crate should ideally use trust-dns, or at least be configurable to do so. On Redox, I believe that trust-dns IS the native API, and using trust-dns also avoids the need for a thread pool.

[carllerche]

Closing due to inactivity and no current strategy.