Merge: Implementing the Nit wrapper over the native postgres wrapper

This wrapper works around the `native_postgres.nit` class and implements the minimal amount of functionality of that class to start a connection, use the execution methods, and inspect the results. It's the next phase in the postgres package, I've also added tests.

@xymus

Pull-Request: #2104
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>