A new streamtube visualization method and different fixes and cleanups for the fvtk module

Hello,

this PR introduces a new visualization method in fvtk for showing streamlines as streamtubes (prettier lines ;) ) and cleanups and fixes some other problems in the fvtk module in order to accommodate the dipy release. There is much more work needed in order to have the fvtk super clean, as it should be, but this is a start towards that direction.

One of the changes which may bother you is that fvtk.red is now fvtk.colors.red. Basically, all colors are under fvtk.colors rather than being at the top. This will make it easier to navigate the more important and available methods. 

Also the fvtk.slicer was simplified a bit to show 3 slices of a volume by default.

Cheers