- merged instances_yields branch r3908:3934, minus the "yield" part which remains...

- merged instances_yields branch r3908:3934, minus the "yield" part which remains slightly problematic
- cleanup of mapper._instance, query.instances().  mapper identifies objects which are part of the
current load using a app-unique id on the query context.
- attributes refactor; attributes now mostly use copy-on-modify instead of copy-on-load behavior,
simplified get_history(), added a new set of tests
- fixes to OrderedSet such that difference(), intersection() and others can accept an iterator
- OrderedIdentitySet passes in OrderedSet to the IdentitySet superclass for usage in difference/intersection/etc. operations so that these methods actually work with ordering behavior.
- query.order_by() takes into account aliased joins, i.e.  query.join('orders', aliased=True).order_by(Order.id)
- cleanup etc.