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

This test tests two pieces: 

 * ability to set branch via constructor
* `for_branch` method.

It might make sense to separate it to two dedicated tests so that each test has single point of failure.