Make DjangoXBlockUserStateClient pass semantic tests

This required the following changes to the DjangoXBlockUserStateClient
semantics:

1) Changes get/get_many to return XBlockUserState tuples, rather
than state dictionaries or (block_key, state) tuples.
2) Raises DoesNotExist if get_history is called on an XBlock that has
had no data saved to it.
3) Returns XBlockUserState tuples as the results of get_history.