given instance is the same instance which is already present.

- merge() now also merges the "options" associated with a given
state, i.e. those passed through query.options() which follow
along with an instance, such as options to eagerly- or
lazyily- load various attributes.   This is essential for
the construction of highly integrated caching schemes.  This
is a subtle behavioral change vs. 0.5.

- A bug was fixed regarding the serialization of the "loader
path" present on an instance's state, which is also necessary
when combining the usage of merge() with serialized state
and associated options that should be preserved.

- The "query_cache" examples have been removed, and are replaced
with a fully comprehensive approach that combines the usage of
Beaker with SQLAlchemy.  New query options are used to indicate
the caching characteristics of a particular Query, which
can also be invoked deep within an object graph when lazily
loading related objects.  See /examples/beaker_caching/README.

- added query_cls keyword argument to sessionmaker(); allows user-defined Query subclasses to be generated by query().
- added @attributes.on_reconstitute decorator, MapperExtension.on_reconstitute, both receieve 'on_load' attribute event allowing
non-__init__ dependent instance initialization routines.
- push memusage to the top to avoid pointless heisenbugs
- renamed '_foostate'/'_fooclass_manager' to '_sa_instance_state'/'_sa_class_manager'
- removed legacy instance ORM state accessors
- query._get() will use _remove_newly_deleted instead of expunge() on ObjectDeleted, so that transaction rollback
restores the previous state
- removed MapperExtension.get(); replaced by a user-defined Query subclass
- removed needless **kwargs from query.get()
- removed Session.get(cls, id); this is redundant against Session.query(cls).get(id)
- removed Query.load() and Session.load(); the use case for this method has never been clear, and the same functionality is available in more explicit ways