Run compiled mapcss code as executable

We can compile the mapcss to a local executable instead of a database function. The following things are needed:

- [ ] a different header and footer
- [ ] increase indentation
- [ ] add a class to emulate [plpy](http://www.postgresql.org/docs/9.1/static/plpython-database.html)
- [ ] document API
- [ ] options to pgmapcss executable to choose destination (db or script)
- [ ] change return values to not encode them to hstore