Backported astropy/astropy#968 including updates for Python 2.5.  Will backport this fix to PyFITS 3.1.x and 3.0.x branches

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2351 ed100bfc-0583-0410-97f2-c26b58777a21
Merge pull request #996 from iguananaut/issue-996

Column.null not used when writing FITS table?
Conflicts:

	CHANGES.rst
	astropy/io/fits/column.py
	lib/pyfits/hdu/table.py
Ignore warning produced by known issue for now; see https://github.com/spacetelescope/PyFITS/issues/44