Condensed gravmag.prism tests into a single func

Looping over field names and using getattr.
Bx component for prism fails in point on vertices

There is a singularity in the formula if the computation point is
aligned with one of the vertices of the prism. The test now uses a grid
that has points that coincide with the prism edges so that it catches
this kind of error.
Prism gravity must be consistent around prism

Implemented a test to check if this passes. Fails for now.
Fix prism around tests and most tests pass

Made safe_atan2 and safe_log functions that correct the arctan2 quadrant
and log(0) limit for the formula. Only test failing now if for tensor
cross-components when points align with the prism edges in some faces.
Fix prism tensor cross-components

There was a singularity when the computation point was aligned with one
the corners of the prism on a certain side. Hacky fix by just moving the
point slightly when that happens. Not the best fit but works reasonably.
Had to reduce the accuracy of tests for these cases.
Removed unused assertions from _prism_numpy

This is just a test module and doesn't need all the assertions for
corner cases.
Clean Makefile and add pip install of required pkg

Try to pip install packages that are need for each target.
Added pep8 commands to Makefile
Added unit test for PEP8 compliance

The goal is to get this test passing.
Removed old test files for harvester script
test dir passes pep8 check
setup.py passes pep8
Added install instructions to README
Removed tesseroid parallel recipe. Took too long
Cookbook passes PEP8
Better test msg for PEP8 check
fatiando package conforms to PEP8 standars
Added the requirements file
Don't tell people to e-mail me anymore
Changelog entry for PEP8 compliance