Added NumberColumn.benfords_law and test. #105

This should close #105. Did most of the work on this a while ago but just remembered I never pushed it!

I separated out the `_pearson_correlation` function as discussed.

In the `benfords_law` function I included the `sign` parameter to specify whether to consider positive/negative or both. Not sure if that's the best way to deal with it, but happy to make any changes.