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.
OPT: Modified dipy.core.sphere_stats.random_uniform_on_sphere, see issue 181.
Merge pull request #180 from matthew-brett/cython-indexing-fix

BF: fix Cython TypeError from negative indices to tuples
RF - fix disperse_charges so that a large constant does not cause the potential to blow up.
BF: Changed from numpy.hstack to numpy.vstack as suggested by MrBago.
DOC: Added a 'Notes' and a 'References' section to dipy.core.sphere_stats.random_uniform_on_sphere, as per Numpy/Scipy documentation style guidelines.
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
BF - fix fit-tensor handling of file extensions and mask=none
Merge pull request #183 from ecaruyer/fix-for-issue-181

OPT: Modified dipy.core.sphere_stats.random_uniform_on_sphere, cf issue #181
Merge pull request #182 from MrBago/disp-charges-constant

RF - fix disperse_charges so that a large constant does not cause the po...
RF - minor cleanup
Merge pull request #190 from MrBago/bf-fit-tensor

BF - fix fit-tensor handling of file extensions and mask=none
DOC: add reference section to ProbabilisticOdfWeightedTracker
DOC: adress comments by Stefan about acronym and reference formatting
Merge pull request #196 from mekman/add_ref

DOC: add reference section to ProbabilisticOdfWeightedTracker