Add `adapt_rgb` decorator and helpers

PR #893 reminded me of a decorator I proposed for handling RGB images. For filters that don't handle volumes but can handle RGB images, this decorator should be useful for a couple of simple cases (applying the filter to the `value` channel of an HSV version or the color image or to each channel of the original RGB image).

I know that "detecting" RGB images is a bit controversial, but sometimes it's the right thing to do. For now, this is intended to be used internally (i.e. functions implemented by scikit-image).

@blink1073: I changed some tests because I wanted to remove some casting and rescaling in `equalize_adapthist`. Let me know if you think those should have remained. See:

https://github.com/tonysyu/scikit-image/commit/d9a39d44eb03dcb647c3e71dff3cc7f755a9d839
Isn't the convention here to use a leading underscore?