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

We use 'course' for so many things, that I didn't realize you meant the key field (v xblock type, v general concept, v as a namespace string). Perhaps say, "Will accept `course` attribute as a deprecated alias for the field `library`"? Would that be clearer to others?