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.
What about we make the GLSL-strings private for now and make people select them via a string name (using `get_colormap`. Then we can move them around as we work out where we want them.