XML serialization: outbound half.
Simplify the HTML in/out: now need an <html> tag.
With the new scenarios scheme, there's no need for empty workbench_scenarios methods.
Rename the UsageStore methods to make a little more sense.
Properly handle HTML content in the text content of the element.
Make usage_store and field_data required to construct Runtime.
construct_xblock et al, can now take an optional field_data to override the one stored on the runtime.
Go back to a classmethod for parsing.
Fix pylint errors