Implement unit testing

This PR implements unit testing with [Mocha](http://visionmedia.github.io/mocha/) and [Should.js](https://github.com/shouldjs/should.js).

I wrote these initial unit tests in a more BDD-approach style, simulating user interaction with the CLI and testing for the expected results. These tests should cover the most basic usage (`harmonic init`, `new_post`, `build`).

As we improve the code and fix bugs, we can (and should) add new tests regarding specific parts of our modules as well.