refs https://github.com/TryGhost/Toolbox/issues/215
- The ContentAPI needs it's own test agent, so we can write e2e tests.
- The main method mostly to be used by the test suites is "authenticate" - it add necessary authentication keys to the request. The agent is not authenticated by default because there are suites that need to test the "non authenticated" requests. Also, there's a need to have the default API key inserted from fixtures level before authenticating (it's not strictly necessary because the key is not dynamic, but I think coupling this point would be a bad move)
