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.
> If you don't like untested code we can completely leave out the authentication part. If someone needs to use wsdl2phpgenerator behind a proxy requiring authentication he can fork it and make it work for the authentication method his proxy uses, as I did with anonymous proxy support in general.

I think that is a fair point as long as we make sure that it is possible to pass authentication values into se system so they are available for future enhancement. I can create a separate issue for this and mention this omission in the docs.