Rename the entropy module and add more to it.

Rename the entropy module to math, and add more functions to it.

The added functions are:

* arithmetic mean
* serial correlation
* monte carlo from pi

This is in addition to the original entropy function.

Each one takes either a string or an offset and length pair as arguments. Each function returns the absolute value of how far the calculation is from the expected value.

More details on these tests are at http://fourmilab.ch/random/. The only one I have not implemented from there yet is the Chi-square test.

This addresses #226.