Added HTTP Client Proxy Support with Basic Authentication

Here's how the proxy client works:

	HTTPClientSettings settings = new HTTPClientSettings(new ProxyClientSettings("http://user:pass@192.168.2.50:3128", ProxyAuth.Basic));
	
	string data;
	
	requestHTTP("http://google.com/", (scope HTTPClientRequest req){},
	(scope HTTPClientResponse res)
	{
		data = cast(string)res.bodyReader.readAll();
	}, settings );

Where `192.168.2.50` has a remote squid proxy server installed and running on port 3128, with the configuration as specified here:

http://howtonixnux.blogspot.ca/2008/03/squid-with-password-authentication.html

to find the htpasswd command, I had to run `yum provides \*bin/htpasswd`, and `ncsa_auth` on my server was called `basic_ncsa_auth`.
I'd really just call it `URL proxyURL;` and remove the property altogether. `URL` could maybe get an assignment operator that takes a string, but writing `settings.proxyURL = URL("http://..");` also isn't really that bad. But this kind of trivial convenience wrappers is generally discouraged in the code (it blows up the API too much).