enh #89, support for SOAP headers in WSDL generator

Parse @header annotations, very similar to normal parameters.

This allows to make authentication and authorization transparent from normal method calls.

This still requires of reading raw POST in beforeWebMethod() and extracting the header tag manually, but at least it's well defined in WSDL and proper error handling can be done.

Headers are processed in same way as body of SOAP envelope is, so if they contain anything the SoapServer will try to call a method named like the top-level tag.
That method can be empty, returning null, but must exist.