Enhancement: Run unit tests on the CLI

In order to improve the way we get feedback from our TDD setup, we
moved the unit tests to a dedicated spec/ folder to be executed by
Jasmine from within a Node.js process. This allows us to not rely
anymore on a web browser for our unit tests, effectively making our
TDD cycle faster.

We use jasmine-node, a Node.js wrapper for Jasmine that gives us a few
helpful features like autotest (`jasmine-node --autotest .`).

What we learned:

* jasmine-node, a Jasmine package and wrapper for Node.js
* How to export functions and variables in a Node.js module
* How to organise specs for Jasmine