Refactor xml_importer.py for easier reading.
Remove post-publish step.
Add make_course_key to modulestores, so that external clients don't need to assume a CourseKey subtype.

Conflicts:
	common/lib/xmodule/xmodule/modulestore/xml_importer.py
Trying to get split to work
Import depth first to help w/ publishing
fixup - re-enable Asset checking.
Move about overview creation to common superclass
fixup! Import depth first to help w/ publishing
Provide workarounds for skip_draft where other code doesn't want its behavior
Support xml blocks w/ multiple parents
Use new check counter & bump count
Support import of ErrorDescriptors.
Use continue version for create course side effects
Disable fetching by version guid for now
More branch override precedence refinement