Refactor client library (#4)

* Add query impls

* Add config module

* Refactor entire client library

We make several changes including:
- A separate `Connection` object
- A special `Empty` class for empty response (to differentiate from
null values)
- Only allow query objects to be sent
- Use a buffer list for all parameters

* Fix query packet generation and update README

Set 0.1.2 release