`boto.mws` - various fixes and code cleanup

This pull request is an attempt to make code of the `boto.mws` more readable and adding some small tweaks and fixes.
So far:
 - added `funtools.wraps` decorator
 - cleaned code of `require` decorator
 - fixed `MWSConnection.create_fulfillment_order`
 - fixed `ListInventorySupplyResult`
 - `ResponseFactory` now can check if call results class is already defined (moved that feature from `api_method` decorator

I'm planning to add more tests (unittests, integration tests).

Please do not merge it - I'm just wanted to get some feedback from other boto developers.