Async support

[Diggsey]

I'm working on integrating this with reqwest to allow sending proxied requests more easily.

It would be useful to add async support to rust-socks:

• asynchronous versions of connect and connect_with_password
• implement AsyncRead and AsyncWrite for the various stream types

[sfackler]

I'd probably want to have a separate tokio-socks crate for that, but seems reasonable to have.

[Diggsey]

Would you mind publishing a new release BTW (with the winapi fix)? I currently have to use a git dependency which will block seanmonstar/reqwest#339

[sfackler]

Done

[elpiel]

I want to join in and help for making this async, but I need some guidance how this should be done.