Garyfallidis fvtk spheres

Eleftherios - I have continued your fvtk_spheres branch, removing the mayavi stuff from viz. There is still one example using mayavi (this one: https://github.com/nipy/dipy/blob/master/doc/examples_0.5/spherical_harmonic_vis_example.py). I assume that it will change quite a bit, because it refers to a lot of functions that no longer exist (MonExpOpdfModel, etc.)? 

Otherwise - if this looks good to you, feel free to merge this stuff into nipy/dipy master