Merge pull request #2484 from mdboom/fits/checksum-failures-32-bit

Fix checksum tests on 32-bit systems.
Conflicts:

	CHANGES.rst
	lib/pyfits/hdu/base.py

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2784 ed100bfc-0583-0410-97f2-c26b58777a21