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
Remove '.' from requirements.txt since:

* It's taken care of in Makefile
* It's taken care of in gittip.py
* Uses '-e .' in those other places
Merge pull request #687 from joeyespo/gittip.py

Rewrite Gittip.py