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.
Remove this file, add a script to package.json

https://github.com/GoogleCloudPlatform/gcloud-node/blob/master/package.json#L38

~~~ sh
 "scripts": {
    "test": "node_modules/mocha/bin/_mocha --reporter spec",
    "regression-test": "node_modules/mocha/bin/_mocha --reporter spec regression/*"
    "cover": "node_modules/istanbul/lib/cli.js cover node_modules/mocha/bin/_mocha"
  },
~~~~

`npm run regression-test` will invoke it. Throw if env variables are not set.