Made the SoapClient class configurable.

There are use cases where one has to use an extended `SoapClient` class. For example if it is necessary to add support for the [NTLM authentication protocol in order to talk to the SOAP api](http://blogs.msdn.com/b/freddyk/archive/2010/01/19/connecting-to-nav-web-services-from-php.aspx).

This commit makes the default `\SoapClient` class configurable through the `Config` class. Needless to say, it defaults to `\SoapClient` to avoid BC breaks in the first place.

The chosen `$clientClass` is used for connecting to the api in the first place, but also for the generated class files.