New exercise on truth tables (and new library for logic exercises)

I've written an exercise on completing truth tables.

Admittedly there isn't a video on this yet, and it's a small jump from the current logical reasoning material, but if you guys want to go in this direction it could make a nice introductory exercise. (In particular, this would combine well with the recent exercise on basic set theory as the beginnings of a discrete math curriculum.)

Two notes of importance:

1. As an efficient means of evaluating formulas, I've added a library called MPL (that I developed for a [separate project](http://github.com/rkirsling/modallogic/)). I apologize if I'm being too forward in including this.
2. There's technically only one exercise type, but formulas are randomly generated from the two variables so that there's either one or two binary connectives and a smattering of negatives (each with low probability).

In general, I've tried to follow the existing examples as closely as possible. I'm looking forward to any feedback you might have.

Thanks!