Power Analysis Module

**Not ready for merge**

The module imperially estimates the power of a dataset using Monte Carlo simulation. The functions take test, a function which accepts a list of sample ids or values, and returns a p-value.

In conjunction with Statsmodels, the code can be used to generate power curves, for a sufficiently large number of samples. (Depending on the effect size, about 20 - 30 in each group).

Pre-merge issues
* Documentation

There are some unused imports. Can you please `import numpy as np`? We avoid importing individual functions as it can sometimes shadow builtins and makes it harder to keep imports and code in synch.