- schema-qualified tables now will place the schemaname

- schema-qualified tables now will place the schemaname
ahead of the tablename in all column expressions as well
as when generating column labels.  This prevents cross-
schema name collisions in all cases [ticket:999]
- the "use_schema" argument to compiler.visit_column() is removed.  It uses
schema in all cases now.
- added a new test to the PG dialect to test roundtrip insert/update/delete/select
statements with full schema qualification