Improving setup.py to fix pip installation

[fabio-t]

Right now, to install this package one has to install numpy before. That's because of import numpy as np which is called even for a command such as egg_info, which pip uses to find dependencies.

This PR accomplishes two things:

• uses setuptools instead of distutils which is "deprecated" (a wrapper to setuptools essentially, if I understand it correctly). This also seems required for the next step (seems like doing this with distutils doesn't work).

• calls numpy.get_include() only when doing actual build/install, not with other setup commands. This allows egg_info to run smoothly and for pip to fetch numpy as a dependency before attempting to build fisher.

[fabio-t]

It should also fix this issue, by the way: https://github.com/brentp/fishers_exact_test/issues/17

[fabio-t]

@brentp just a gentle nudge :) have you had the time to look at this?

[brentp]

sorry. I completely missed this. Thanks.

[fabio-t]

@brentp thanks!