Big refactoring.

 * Get rid of Jsonify. It was too clever for my own good.
 * Created HttpClient abstraction, so possibly other HTTP libraries
   could be used.
 * Docs, tests, better code coverage.
 * Abstracted authentication to an interface, restricted to a particular
   hostname.
 * Got rid of the client.apis.resource indirection; now it's just
   client.resource.