Rewrite Gittip.py

* Windows support
* Remove fabricate
* Include a docstring
* Create local.env from default_local.env
* Only execute each step when specific files are missing
* Use \_\_name__ == '__main__' convention for potential reuse
* Have it check for 'pg_config' sooner for cleaner error message

This change depends on https://github.com/zetaweb/www.gittip.com/pull/683.