New locators for content libraries

This PR implements two new types of locators necessary to facilitate content libraries. I'll be posting a separate PR to edx-platform for the content libraries prototype soon, but for now I'd like to start the review process for this locator code.

The new locators are described in detail under the "New Locators" heading at https://openedx.atlassian.net/wiki/display/SOL/Content+Libraries

No - `CourseKey` declares them as `@abstractproperty` and the OpaqueKey code sets them using `setattr` during instantiation - so as a result, there is never a concrete declaration of these fields anywhere in the inheritance chain except here... without these lines, I was getting a whole bunch of pylint warnings.