add "expired" functionality

[Mike Bayer]

we have a function now to "refresh" an object:

   objectstore.refresh(obj)

  or

   objectstore.get_session().refresh(obj1, obj2, obj3)

add the next step, which is "expire":

   objectstore.expire(obj)

the next attribute access results in a call to refresh() before returning the attrifbute.

[Mike Bayer]

added + unit test for both refresh/expire in changeset:1126