Fvtk cleanup

* I've cleaned up how colormaps are handled a little in dipy, and moved the actual colormap data information from the code into a json file. This makes the code cleaner and makes it easier to handle matplotlib colormaps. All matplotlib colormaps can be used in any part of fvtk if the user has matplotlib installed

* I've replaces all the if/elif/elif calls in fvtk with a new function

* Added tests, and fixed a bug I discovered while writing said tests, for colormaps.

This should supplant #376 and #377 I believe.