Add other types of statistical testing for a second opinion

[emtwo]

This could include permutation tests and/or Mann Whitney U. Perhaps we can use Rob's new tests here: https://github.com/mozilla/python_moztelemetry/blob/master/moztelemetry/stats.py#L81

[emtwo]

Note that the link above is an implementation of Mann Whitney U for histogram-shaped data. For data in the form of arrays of samples, we can directly use the scipy library: https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.mannwhitneyu.html