NF added class to save streamlines in vtk format

This is a class that provides to save streamlines (as generated e.g. with eudx) in the vtk file format. 
Some features are:
  - setting an affine transform to save the streamlines in the 
    data's coordinate system (rather than voxel coordinates)
  - writing in ascii or binary file format
  - saving colored streamlines for the local or global direction of the streamline
  - decorating the streamlines with one value for every streamline (e.g. the streamline's length)
  - decorating the streamline with a value for every vertex (e.g. the FA values)
  - colors and decorations can be viewed e.g. with paraview (http://www.paraview.org/)
  - errors are indicated by return code, no exceptions are thrown (or should I use exceptions?)

I am relatively new to python. So this is my initial project for dipy and a thorough review might be required.