Use flake8 and pylint instead of pep8 (fatiando#382)

flake8 is better than pep8 and easier to configure.

Added a "make lint" command to run pylint static analysis.
This won't break the build because there are so many failures
that it will take a long time to sort through.

Added "make check3" command to use pylint to check Python 3
compatibility. This will break the build if something doesn't pass.
It will make it easier for new code to be ported to Python 3.
I put in the effort of making the necessary adjustments for these
checks to pass.

.flake8

@@ -0,0 +1,5 @@
+[flake8]
+exclude =
+    fatiando/_version.py,
+    fatiando/_our_duecredit.py
+ignore = F401,E226