Enable (and test) xml export of pure XBlocks

[LMS-179]
[LMS-209]
[LMS-1345]
Switch test_xblock_wrappers to use ddt, and to considate logic
Refactor test_xblock_wrappers to use Factories
Add tests of XML exporting to test_xblock_wrappers
Merge pull request #2221 from cpennington/xblock-xml-serialization

Enable (and test) xml export of pure XBlocks