Use lut for colorconv

This addresses #1133.  Prior to the change, @stefanv's example took ~0.09sec on my machine, and after we get the following behavior:

warmup: 0.190999984741
subsequent: 0.0569999217987

Not a blazing speedup, but nothing to sneeze at.  This could be further improved by storing the LUT on disk.  What is the mechanism for that?  I see there are `.npz` files in `skimage.data`, but no script that originally created them.