Demeter adding and removing responses for `TestClient`

This demeters adding and removing responses for `TestClient`s.
This avoids demeter violations that were commonly used like
`client.responses.add`. This is to avoid issues if we decide to
change how the responses are accessed or other parts of their
interface.
Merge pull request #36 from redding/jcr-demeter-test-client

Demeter adding and removing responses for `TestClient`