enable SCROLL / NO SCROLL for named cursors

Thank you, it seems an interesting patch.

Would you mind adding tests and documentation too?

It would be great if you could fork my git repos https://github.com/dvarrazzo/psycopg and push your changes there for integration. Note that in my "devel" branch I've pushed changes that may probably conflict with your patch: you may want to create a 'scrollable-cursor' branch from "master": I can care about merging everything.

Thank you very much.

Got no reply back. However I've merged the patch and added test and docs myself.

I've made scrollable three-states: None (default) specifies no scroll option. True/False specify SCROLL/NO SCROLL. No option and NO SCROLL are slightly different and the default ensures current behaviour is kept.