Luv colorspace

From [this conversation on the mailing list][convo], Issac Gerg inquired about including conversions to and from the CIE Luv colorspace in the `color` module.  I started on my own implementation, incorporated changes from Issac and created a series of tests that mirror those used for the CIE Lab color space conversion functions.

[convo]:    https://groups.google.com/forum/#!topic/scikit-image/DIRaSXJoEes

This is my first pull request to scikit-image.  Please let me know what I can do to make the submission more appropriate for inclusion.