Documents PortType->Interface for the clientInterface

For me using the PortType in the clientInterface property didn't work resulting in a not found class when trying to convert the property value from a String to a Class. Changing it to the Interface instead worked nicely.

It may be that PortType should be replaced by interface in the example OR that there is some other kind of WSDL that matches the PortType scenario. 

This pull request may be a solution to issue #43.