- added has(), like any() but for scalars

- added has(), like any() but for scalars
- added **kwargs to has() and any(), criterion is optional; generate equality criterion
against the related table (since we know the related property when has() and any() are used),
i.e. filter(Address.user.has(name='jack')) equivalent to filter(Address.user.has(User.name=='jack'))
- added "from_joinpoint=False" arg to join()/outerjoin().  yes, I know join() is getting a little
crazy, but this flag is needed when you want to keep building along a line of aliased joins,
adding query criterion for each alias in the chain.  self-referential unit test added.
- fixed basic_tree example a little bit