WIP: Add greyscale and color-blindness filters.

Any artist can now do:

`x.set_agg_filter(...)`

to set a greyscale or color-blindness filter.  This is the first step toward a more automated previewing for colorblind accessibility of plots.

Only works with Agg backend, of course -- this just builds on top of the work @leejjoon did.

@kthyng: I'd love your feedback on this, as it was really inspired by your Scipy presentation.