Added proxy option to ctx

Hi, Yaron!

We're using ws.js in a project and so far we've found it to be the easiest way of accessing Web Services from a Node.js application.
  
However, one of our partners came up with a (strange?) requirement: their server will only accept requests coming from a static IP. Therefore, we have to use a 3rd-party service called [QuotaGuard Static](https://www.quotaguard.com/static-ip), which simply acts as a proxy in our case. In order to do so, we've had to slightly modify ws.js to accept another ctx option called `proxy` and simply pass it to the underlying [request](https://github.com/mikeal/request).

The only relevant change is in `lib/handlers/client/http.js`, but I've also modified `README.md` to document the option.

Please consider accepting this pull-request, since I believe there may be other people out there confronting with the same issue.

Let me know if you have any questions.

Thanks!