Cursor methods don't support back_as argument

[Changaco]

Postgres.all() and Postgres.one() support the back_as argument, but the same methods on cursors don't, which for example prevents doing (cursor or db).all(..., back_as=dict).

[chadwhitacre]

Currently we're depending on psycopg2's different cursor types for the implementation of back_as. I supposed we'd need to unroll those in order to support a back_as argument to cursor.{one,all}. Python's namedtuples have an _asdict method that can be used to serialize to a dictionary. Maybe that is helpful here?