RFC-24, implementation of item 3.2: parent references in PYTHON

[Umberto Nicoletti]

To the python mantainers:

I have resumed my work on rfc-24 and I'm now tackling item 3.2.
The purpose of item 3.2 is to prevent early garbage collection
when a parent reference goes out of scope, the details are at:

http://mapserver.gis.umn.edu/development/rfc/ms-rfc-24/

I have already committed the necessary changes for Java mapscript and then
I have prepared a patch for python that you can find here:

http://trac.osgeo.org/mapserver/attachment/ticket/2442

The patch introduces:
- a test program (tests/cases/parentreference.py)
- modifications to the wrappers of classObj and layerObj so that the
test program passes

Before making any changes I'll wait for your review/approval.

Please note that the patch must applied from mapscript/python.

Best regards,
Umberto