Colormaps

This implements colormaps as per https://github.com/vispy/vispy/issues/414 . It is a first pass that uses OpenGL functions. Visuals will need to integrate this, but it has already been done for the Line visual. It includes a new example 'colored_line.py' that cycles through all the colormaps.
Adding color function support is a good idea, but this line makes it almost useless.. 
Perhaps that can be left for a future PR though, after some discussion about how we really want this API to look.