Support schemas and quoting in DB() and pgdb

Since commit f82d4cd, inserttable() escapes the table name, which interferes with use of schemas.
We talked about this in DBAPI2 in issue #47

The table could be passed as a tuple of (namespace, table) or (table,) or (db,namespace,table), which must be individually escaped. Or take a kwarg which would indicate that the table is already quoted (and might contain schema).

Mixed-case names and special chars should still be supported.