Add `image_label2rgb`
Fix: Add setup.py which adds data file to the build
ENH: Add some checks on the inputs.
TST: Add tests for image_label2rgb
DOC: Change more examples to use `image_label2rgb`
API Change: `image_label2rgb` changed to `label2rgb`.

The name change reflects a switch in the argument order (and makes the image an optional argument).
Change color definitions from ini file to python module

This simplifies the code and allows users to use either `color_dict` or module
variables, e.g., `rgb_colors.white`.
Merge pull request #485 from tonysyu/image_label2rgb

ENH: Add `label2rgb`.