Docs require skimage

Three examples now require it, for the rgb2lab function:

```
users/plotting/colormaps/lightness.py
5:from skimage import color

users/plotting/colormaps/Lfunction.py
9:from skimage import color

users/plotting/colormaps/grayscale.py
6:from skimage import color
```

We should probably just subsume rgb2lab -- skimage is a fairly heavy dependency just for this.
