Use protocol buffers to get the response and error type values

[danielcompton]

Using the protocol buffer definitions for response and error types makes the code more self documenting, and less prone to error when transcribing response or error types.

[danielcompton]

There's a problem with using Java enum's in case statements. There is a solution at http://stackoverflow.com/questions/16777814/is-it-possible-to-use-clojures-case-form-with-a-java-enum, but it's pretty ugly, and probably not worth the extra complexity.