Fixes a double-free bug in find_all_wcs.  If one of the headers raises an exception with wcsset, the wcs gets free'd twice because it's already in the list and we were also decrementing the item directly.  (PyList_SetItem "steals" a reference).
Add data file
Another bugfix -- find_all_wcs wasn't converting the header string correctly.
Python 3 fix
Merge pull request #747 from mdboom/wcs/double_free

Fix a double-free exception in `astropy.wcs.find_all_wcs()`