Merge pull request #496 from edx/feature/cdodge/add-a-mixed-modulestore

Add a 'MixedModuleStore' which aggregates between XML and Mongo Modulestores
Remove 'edu' from examples

per our agreement not to use the domain
Location to Locator mapping utilities

partial impl
pep8 violation fix (shadowed identifier)
Translate locations <-> locators

unit tests and fixes they implied
Enable test via naming it correctly
Require edx:// for urls

But don't use this parser for urls embedded in a browser url request.
Implement location mapper as stand-alone helper class

with own db connection.
Move singleton accessor w/ django dependencies to same place as modulestore() singleton accessor
Clarify doc/comments on default v explicit mappings
Raise ItemNotFoundError rather than return None if no map found
Use a sort of name to choose if > 1 map
and some minor comment changes.