Base classic API on DB-API

[Cito]

The idea here is to divorce the classic API from the C module and make it a wrapper around the DB-API module. If done carefully then the classic interface can work with any DB-API compliant module.

(copied from Trac ticket 36, created 2010-09-12)

[Cito]

The problem here is that the classic API is a mixture of high-level methods and low-level wrapper methods for the pqlib. The approach here would only work for the high-level methods.