This adds DefaultReviewer as a resource. It doesn't cover the full

featureset of the API, but does allow for basic interaction with the

resource (mainly, basic fetching and deletion).
This isn't used today in Review Board, but may be useful down the road,

and is being used in RBCommons. I've opted to add it here, since its

size is minimal, and it helps complete our JavaScript API support, which

will only help extensions.

Unit tests pass.
Used this in RBCommons to load and delete.