For scripts, use the Python interpreter that's found on the path.

Not everyone has `python` installed in `/usr/bin` (in particular, on FreeBSD/NetBSD/etc it would likely be in `/usr/local/bin` -- and in [my own dogfood](https://github.com/catseye/toolshelf) it could be anywhere.)  So, this change makes the hashbang lines use the `python` found on the search path, using `/usr/bin/env`.  I don't know if this is considered "best practice" exactly, but it's fairly widespread and in my experience it's a pretty good practice.