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.
We talked about having a repo for these sorts of things like `vispy.shaders` or so. I agree we should move these to a `_colormap_glsl.py` file for now so if we later decide to move it to `vispy.shaders` it's just a `git mv`