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.
That's true, there is no proxy_scheme option in SoapClient. I only introduced this key to keep the scheme parsed from a given URL string when building the proxy string to pass to the libxml stream configuration. I did this because in the comments in the php manual I saw a lot of examples using a "tcp://" url for proxy configuration. But I don't know whether this is really necessary because for me it works without the scheme using only "hostname:port" as proxy URL.