Documentation Improvements

Preview:
http://www.stsci.edu/~lbradley/photutils/
The scikit-image function `label2rgb` requires the image to be a 3D `(M, N, 3)` RGB image.  Often when I think something from `scikit-image` could be useful, I'll find that it's not quite applicable (e.g., `image` is usually required to be `uint8`, or RGB(A), etc.).