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.
should we put this list of GLSL functions in another file (like `_colormap_glsl` or something)? @Eric89GXL @lcampagn WDYT?