Expose Statement.columnNames

7fa9a57cee70cbd9a06eea9531377b19c9447581 removed Statement.values, which
provided dictionary access to a row. It was removed to avoid bugs when
joining tables with ambiguous column names. Developers should have the
ability to introspect enough to extend Statement themselves, though,
should they so choose.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>