Add IndexedStackColorMapper for ImageStack glyph about bokeh

[bryevdv]

I should have re-read the comments in the old PR, since there are some design wrinkles we need to fix up:

One problem with this approach is that both the LinearColorMapper and IndexedStackColorMapper must have a palette specified (None is not allowed for ColorMappers) but only one is used.

Also, I have had to reimplement part of ContinuousColorMapper._v_compute within IndexedStackColorMapper when it scans all of the data but only displays one 2D slice of it. Probably a better approach would be a refactor of the various _v_compute functions to accept the scanned data rather than scan it themselves.

I definitely think it is better to be able to have a "color mapper" that can be configured with and delegate to another colormapper. So we will need to introduce a new class level and have the base class not take a palette, so that there are not duplicative palette values.

I also agree that scanning needs to be improved and generalized, e.g. ideally it would be good for this case to cache scan results for every index and return only the ones for the current index.