Extend instance method ActivationKey.path

Allow a client to request the "releases" path from an activation key. This path
is in the following format:

    /activation_keys/<id>/releases

Add tests for this new functionality. Add the newly-created module of tests to
the API documentation.
Add tests targeting BZ bug 1112717

Check whether it is possible to create an activation key and fetch its releases.
Merge pull request #1147 from Ichimonji10/bz-1112717

Create test targeting BZ 1112717