Test coverage cleanup

Here's the first cut at adding some test coverage and cleaning up the ruby client:

- Removed Database class (isn't used anywhere)
- Added simple tests for series list and reading by key
- Added some basic http response json fixtures. We could go with something like VCR to capture responses, but I'm usually against bringing in that kind of dependency unless we really need it. Thoughts?
- Broke out domain objects into separate files
- Cleaned up some non-idiomatic ruby code