Add the six module to a pyfits.extern package.  This is version 1.5.2 of the module; its license is included in the source file.

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2492 ed100bfc-0583-0410-97f2-c26b58777a21
Update all PyFITS source files to a) Use relative imports where possible (there are a few cases where they won't work on 2.5) and b) Use bits from six to improve cross-Python compatibility.

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2493 ed100bfc-0583-0410-97f2-c26b58777a21
Get rid of bits that mess with builtins uninvitedly

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2494 ed100bfc-0583-0410-97f2-c26b58777a21
Mention cross-Python unification in the changelog

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2495 ed100bfc-0583-0410-97f2-c26b58777a21