Add missing features from Header.insert that were intended to be there ever since version 3.1.  See spacetelescope/PyFITS#12

Conflicts:

	CHANGES.txt

git-svn-id: http://svn6.assembla.com/svn/pyfits/branches/3.1-stable@2526 ed100bfc-0583-0410-97f2-c26b58777a21