Implemented gamma coefficient, cleaned up docs

Big disclaimer: this is my first contribution to an open source project, so I could very easily have done something wrong. 

I noticed that the [gamma coefficient](http://en.wikipedia.org/wiki/Goodman_and_Kruskal's_gamma) hadn't been implemented yet, so I went ahead and did so. 

While I was doing that, I noticed that the "concordant?" function would fail silently if you passed it a list with more than two pairs. Consequently, I added a test to check if there are more than two pairs passed to it.