Fixed the issue reported by James Turner, that consecutive calls to writeto on the same HDUList could result in corruption on subsequent calls.  This turned out to be the same issue that issue #40 was a symptom of, so this takes care of them both in one swoop.  The way HDU objects manage file offset information still needs to be totally reworked but this is a reasonable fix for now.

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2745 ed100bfc-0583-0410-97f2-c26b58777a21