Fix soap12:determine_soap_action
Raise an usefull exception on field not found

Raise a value error with the class name and the fieldname
when the field is not found by the method `ComplexType._find_field`.
Clean Django dispatch
Add python3 compatibility to some examples
ability to use custom soap headers.

The code was written mostly by numion <numion.oss@gmx.net> and ported by Thomas Recouvreux.
(Felix Schwarz: Licensed under the original soapbox BSD license as per comment in private email)
Use custom headers in dispatcher
Add ws-addressing
Factorize soap12.py & soap11.py
Update doc
Move SoapboxRequest & SoapboxResponse to core.py
Update SOAPError & move it to core