When calling 'python setup.py' use the same PYTHONPATH as the script.

See issue #24.
Use os.sep instead of hardcoded colon. Use PYTHONPATH only on posix.
Add PYTHONPATH in environment when using sys.executable.
Remove earlier PYTHONPATH tries on this branch
Merge pull request #56 from zestsoftware/maurits-issue-24

When calling 'python setup.py' use the same PYTHONPATH as the script.