Replace execfile with something that works under both Python 2 and 3

Execfile doesn't exist in Python 3, this commit replaces the use of
execfile with an equivalent construct known to work under both versions
of Python.

The change is actually the result of running 2to3 over setup.py, I just
added a note to explain its presence.
Merge pull request #99 from lvillani/python-3

Replace execfile with something that works under both Python 2 and 3