Make setup.py Py3k- and PEP8-compliant.

Replaced the call to execfile (does not exist in Py3k) with runpy (introduced in
Python 2.5). Also made some stylistic changes so that the file conforms to PEP8.
Merge https://github.com/pmneila/PyMaxflow

Conflicts:
	setup.py
Fixed mistake in the merge

I joined the two lines by mistake.
Merge pull request #6 from josipd/master

Make setup.py Py3k- and PEP8-compliant.