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.
As I said in an earlier comment, out proxy does not require authentication and so I don't know whether it works or not and can not test it. The code for the proxy_authentication http header is basically taken from http://php.net/manual/de/function.stream-context-create.php#111032, but I don't know with which proxy configurations it works and with which not.

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.