- tightened down conditions used to locate "relation direction", associating

- tightened down conditions used to locate "relation direction", associating
  the "foreignkey" of the relationship with the "primaryjoin".  the column match now
  must be exact, not just "corresponding".  this enables self-referential relationships on a
  polymorphic mapper.
  - a little bit of improvement to the concept of a "concrete" inheritance mapping, though that concept
  is not well fleshed out yet (added test case to support concrete mappers on top of a polymorphic base).