Add proxy authentification support for the RestClient

Hello,

In the current state, it is possible to specify the host and port of the proxy, but not the username and password that are requested. It would be useful to be able to specify the user and password to consume a webservice when your network is behind a web proxy.

I found a workaround, but I requires some work ... http://stackoverflow.com/questions/3023885/consume-restful-webservice-through-web-proxy

Great job so far, it is very helpful !