Code review changes re split import/export, create_xblock

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/tests/test_crud.py

common/lib/xmodule/xmodule/modulestore/__init__.py

...on/lib/xmodule/xmodule/modulestore/draft_and_published.py

common/lib/xmodule/xmodule/modulestore/mixed.py

common/lib/xmodule/xmodule/modulestore/mongo/base.py

common/lib/xmodule/xmodule/modulestore/split_mongo/split.py

...ib/xmodule/xmodule/modulestore/split_mongo/split_draft.py

...modulestore/tests/test_cross_modulestore_import_export.py

common/lib/xmodule/xmodule/modulestore/xml_importer.py

common/lib/xmodule/xmodule/tests/__init__.py