BF: fix Cython TypeError from negative indices to tuples

It looks as though Cython now raises a TypeError if you use negative indices
with the boundscheck decorator, even if these indices are to tuples.

Use positive indices instead.
Merge pull request #180 from matthew-brett/cython-indexing-fix

BF: fix Cython TypeError from negative indices to tuples
DOC: replace soureforge.net links with nipy.org

Change canonical nipy web address.
Merge pull request #185 from matthew-brett/no-sourceforge-links

DOC: replace soureforge.net links with nipy.org