New XML parsing machinery.
UsageStore abstract interface
Use the new Plugin injection instead of @patch
XML serialization: outbound half.
Simplify the HTML in/out: now need an <html> tag.
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