- Improved query.join() such that the "left" side

- Improved query.join() such that the "left" side
can more flexibly be a non-ORM selectable,
such as a subquery.   A selectable placed
in select_from() will now be used as the left
side, favored over implicit usage
of a mapped entity.
If the join still fails based on lack of
foreign keys, the error message includes
this detail.  Thanks to brianrhude
on IRC for the test case.  [ticket:2298]