merged the polymorphic relationship refactoring branch in. i want to go further...

merged the polymorphic relationship refactoring branch in.  i want to go further on that branch and introduce the foreign_keys argument, and further centralize the "intelligence" about the joins and selectables into PropertyLoader so that lazyloader/sync can be simplified, but the current branch goes pretty far.
  - relations keep track of "polymorphic_primaryjoin", "polymorphic_secondaryjoin" which it derives from the plain primaryjoin/secondaryjoin.
  - lazy/eagerloaders work from those polymorphic join objects.
  - the join exported by PropertyLoader to Query/SelectResults is the polymorphic join, so that join_to/etc work properly.
  - Query builds itself against the base Mapper again, not the "polymorphic" mapper.  uses the "polymorphic" version
 only as appropriate.  this helps join_by/join_to/etc to work with polymorphic mappers.
  - Query will also adapt incoming WHERE criterion to the polymorphic mapper, i.e. the "people" table becomes the "person_join" automatically.
  - quoting has been modified since labels made out of non-case-sensitive columns could themselves require quoting..so case_sensitive defaults to True if not otherwise specified (used to be based on the identifier itself).
  - the test harness gets an ORMTest base class and a bunch of the ORM unit tests are using it now, decreases a lot of redundancy.