Add backend unit testing framework

[andrewmarklloyd]

Let's get a framework and start adding some unit tests for the backend. Please add some comments on which framework would be a good fit for Podsheets

[gnoejuan]

My opinion is the framework should be the same for the front end and the back end. I do believe Mocha is capable of handling both. While Mocha does use Node's assertion functions, there are other assertion libraries available. My memory leads me to believe that Chai.js was usually recommended on most, if not all, Mocha tutorials.