Add proxy support

Added proxy support and a test to support it works, it's basically a copy-paste from the test linked by @marcosnils  from node's request library
It's heavily based on @giefferre work.

I've created another client and transport, initially empty, that are initialized and used whenever a request specifies a Proxy.
Default transport now inherits proxy environment variables.

Cheers!
I know it's not very likely to happen and I don't think it represents a problem. But we need to take into account that it may happen.