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. 
That difference comes from a sum of squares within the `pearsonr` computation (it produces "x" in one ordering but  next_float_after(x) for the other). Numpy uses a naive sum algorithm, and there's been some discussions about adding a more [precise sum](https://github.com/numpy/numpy/issues/2448) but nothing happened. Better options include e.g. [pairwise summation](http://en.wikipedia.org/wiki/Pairwise_summation) (small time penalty, little error) and the [kahan algorithm](http://en.wikipedia.org/wiki/Kahan_summation_algorithm) (larger time penalty, smallest error). Any way, I don't know the effects of this small difference in the test...