Make pyloxi inheritance and list handling generic

Reviewer: @andi-bigswitch

A few major pieces:
 - Make module (action, instruction, message, etc) templates identical and then consolidate into a single template.
 - Replicate the OpenFlow class hierarchies in the generated Python code.
 - Rewrite parsing code as automatically generated methods on virtual classes.
 - Handle length members inside unpack methods, removing the need for hand-written list deserializers.