#75 Add support for WS-Eventing

[po79]

WS-Eventing can be implemented above gSOAP, but it is not very fast. If you have multiple subscriber, you have to serialize the message for each subscriber. Different optimization approaches exist since the message body is always the same (only the SOAP header differs):

1. cache the length of the SOAP body in case of two serialization (when no buffering is activated)
2. cache the whole SOAP body

A general approach for an API would be, to add code that sends a message to multiple endpoints (as required in WS-Eventing with reference parameters etc.)