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.
Perhaps this and the above could be reduced to something like this.

```php
$proxy_value();
foreach(parse_url($value) as $key => $value) {
   $key = strtr(array('pass' => 'password'), $key);
   $proxy_value[$key] = $value;
}
$value = $proxy_value;