Remove null handlers

The null handlers blocked all loggings
use dict as TypeRegister + add warning for duplicates
fix code_generation
Django dispatcher extending SOAPDispatcher
Fix maxOccurs type

maxOccurs is now able to accept either an int or 'unbounded'.
small test cleanup for MaxOccurs/XSD spec element
organize imports
fix maxOccurs for ListElement (off by one)
rephrase exception when trying to too many items to a list (maxOccurs)
extract TypedList from ListElement
use guard clause in TypedList to make the code more readable
Preserve elementFormDefault attribute in Schema

(Felix Schwarz: Licensed under the original soapbox BSD license as per comment in private email)
remove the exception handler from SOAPDispatcher (introduced in ab6be5a4) because the simple solution breaks debugging middlewares