- got self-referential query.join()/query.outerjoin() to work.

- got self-referential query.join()/query.outerjoin() to work.
- PropertyLoader adds local_side set which is the opposite of remote_side, makes
the self-referential aliasing a snap.
- added "id" argument to join()/outerjoin() to allow access to the aliased joins
in add_entity().
- added "alias" argument to add_entity() to get at entities selected from an explicit
Alias
- starting to move EagerLoader.AliasedClasues to a general utility function which
will be used by Query as well