Proxy configuration

Add a "proxy" configuration key for setting the proxy on both SoapClient and libxml for fetching the WSDL. The proxy can be specified either as a string ("host:port") or as an array containing 'host', 'port', and optionally 'login' and 'password' keys.

Honestly, I could not test whether the proxy authentication with login and passwort really works because I don't have a proxy that requires authentication. Without authentication the proxy settings work well for me.

Initially I developed these changes against the 3.x branch but since it got merged to master I rebased on master and created a new pull request.
Sorry. I was not being clear and primarily commenting on my own suggestion.

Throwing an exception when `soapClientOptions['proxy_*']` is set would break backwards compatibility - not this.