Adding regression tests.

Currently coverings for datastore.

@rakyll if the approach is ok I'll add to the README instructions on how to run the tests (basically configuring the export variables in run_regression_tests.sh) and I will add the tests to our build.
You can set the timeout as a argument to _mocha.

~~~ sh
node_modules/mocha/bin/_mocha --reporter spec --timeout 4000 regression/datastore.js
~~~