Message2485

Patch to address 2 items:

 - expose __get__()
 - check that arg to classmethod is a callable

The relevant test case is classmethod() in test_descr
(from cpython 2.3), which now passes.

It would probably be useful to take a more rigorous
approach comparing exposed attributes from the builtins
between jython & cpython since the test-cases probably
don't catch everything. However hopefully taking small
steps is still progress.

Needs review. Thanks.

- kz