Misc. cleanup

I've done a few bits of cleanup in the codebase, and thought you might be interested in some/all of the patches. I can remove any of them that you're not interested in.

- Tag model struct fields. Instead of a bare struct field tag, use the standard `key:"value"` tag format with a key of `riak`. This allows model structs to be used with other packages that need tags (like `encoding/json`) easily.
- Removed a bunch of debug printing from the tests.
- Move the protobuf code into a subpackage `pb`, this makes the godoc output much nicer.
- Move the custom json code into a subpackage `json` so that the Marshal/Unmarshal functions and related types/variables don't get exported from the root package.