[WIP] Optional schema

[tdamsma]

Little experient to see if the schema parameter could be made optional. As the schema is the first argument it can;t be changed to optional without breaking backards compatibitily or changing all arguments to kwargs. So for the moment I just added support for accepting an empty string as schema.

For the experiment, I only added support to the PGFunction. If there is agreement on how to do this I can extend this to all other classes