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