Adjust parameters to construct_xblock_from_class to match XBlock repo.
Minor cosmetic code fixes.
Use XModuleDescriptor.parse_xml to interpret XML.
Update XBlock requirement to new XML code.
Get rid of all mention of XModuleDescriptor.
Fix the new tests to use the new way to make XBlocks.
We aren't ready to use XBlock.load_class, there are duplicate entry point names, and simply choosing the first one leads to unpredictability.
Merge pull request #1100 from edx/ned/new-xml

New XML infrastructure for XBlocks