Support soap headers

Hi,
This adds attributes to the operation that support the defined soap header from the WSDL.  

The headers are added to the envelope at the time it is built.  Currently it doesn't support more than one header.

I'm not convinced that the interface is the nicest but it definitely works for me.  I've tested it against the Remedy webservice that that generated the WSDL and it works fine.

All good.  I'm going to try and use this in production anyway :)