Add Hommola cospeciation test

Here, we have added a statistical test for host:parasite cospeciation derived from Hommola et al (2009, Molecular Biology and Evolution). This is added as a new module called evolve under math.stats.

We are looking for feedback on the organization and naming of the evolve module. We're not quite sure what to add to the top level description for this module, so any other ideas would be appreciated. 

Note: the test itself is using numpy arrays rather than distance matrix objects because we are using the positional relationships between host, parasite, and an interaction matrix rather than the labels. 
So this is interesting: I think because the exact nature of the permutation function is not identical between the two tests, the observed p-values returned are not identical, even when given the same seed.  However, when I run each test 100 times (999 permutations), seeding with the first 100 integers, the distributions of returned p-values are identical (sd = 0.00416, 0.00406; p = 
0.01718, 0.01712). 

Any thoughts on how to unit test this?