Add tests for exec against in-memory H2 database

Hey Taylor,

This pull request adds some basic tests for the `exec` method. Notable changes:

* The H2 driver has been added as a development dependency
* Each test in `oj.core-test` is now run in the context of a connection to the in-memory H2 database
* Added tests to the `exec` function; basic CRUD operations now covered

Take care,

Erin