More docs, cleaner, stricter code

• Refactor codegen.py
• Revert the behaviors added in 3.0.0a2 with regard to documented defaults and None
• Use amqp0-9-1.extended.xml instead of amqp-0-9-1.xml to get the documentation for RabbitMQ added classes/methods
• Add strict value checking for deprecated values
• Remove empty __init__ functions from method classes