Add dialer interface according to golang.org/x/net/proxy.

[jackyyf]

This allow us to use shadowsocks as upstream proxy in go program more easily, and a (like-to-be) standard interface would be nice.

I've attached some testcases with pull request, and all tests passed with forked version.

[riobard]

I actually regret to have the core providing the helper functions since they assume too much of the use case. I was hoping the repo stays barely minimal so others could easily embed it into their own projects. Do you think the dialer is absolutely necessary given the minimalism philosophy?

[jackyyf]

You are right, it could be moved to another separate package, I'll refactor my code, thanks for pointing that out :)

[riobard]

I think your code could be organized into a standalone package providing proxy support via e.g. Shadowsocks, SOCKS5, HTTP proxy, etc. That will be more compatible with the design of golang.org/x/net/proxy. Let me know if there's anything I can do to help.

Also, I'm thinking about separating the reusable packages and the demo server/client to use them.

[jackyyf]

I've moved dialer interface to a separate package named proxy. I don't know if this is ok, and please comment for change if this name is too generic or too ambiguous.

[riobard]

The code looks pretty good now. What do you think if you make it available as its own repo on GitHub?

[jackyyf]

Sorry for the looooong delay, I had some serious about my graduation design and it took whole June to get around with it, and I just have spare time now to reply this issue.

Making as own repo is fine, but I would recommend to include this within primary repo, or at least as a project under shadowsocks organization to make this interface used by more people. Anyway, thanks for the review :)