Initial implementation of test suite for npm registry implementations

The goal of this module is to provide a test suite that checks whether an alternative registry implementation is compatible with the official npmjs registry. By keeping the tests outside of any implementation, we can use it later to test various components of our private registry solution.

To ensure the tests are up to date, `npm test` starts a Couch DB instance, uploads the official npmjs registry app and run the test suite against this registry.

/to @sam-github I suppose you are the best person to review this code. If you think there is no need to review this, then I am fine with that too.