These are notable changes in XBlock.
- Use tox to test against Django 1.8 and 1.11.
- Add quality testing via tox and pylint.
- Make Scope enums (UserScope.* and BlockScope.*) into Sentinels, rather than just ints, so that they can have more meaningful string representations.
- Rename export_xml to add_xml_to_node, to more accurately capture the semantics.
- Allowed Runtime implementations to customize loading from block_types to XBlock classes.