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:

- [x] a different header and footer
- [x] increase indentation
- [x] add a class to emulate [plpy](http://www.postgresql.org/docs/9.1/static/plpython-database.html)
- [ ] document API
- [x] options to pgmapcss executable to choose destination (db or script)
- [x] change return values to not encode them to hstore