check for python version <3

In some unices (Arch for instance), the default python is python3, so the system chokes when trying to install bitmask. Even though we specify the supported versions in the classifiers, we should add a specific check to warn user of the python3 incompatibility at the moment.

This should work:

import sys
if not sys.version_info[0] == 2:
    print "Sorry, Python 3 is not supported (yet)"
    exit()

setup(...

(from redmine: created on 2013-09-04, closed on 2013-09-04, relates #3710 (closed))