Luv colorspace conversions

Adding XYZ <--> Luv, RGB <-->
Adding test functions to mirror Lab tests
Adding references regarding Lab and Luv spaces to docstring
Removing unneeded division checking code
renaming machineEps to eps to match convention
flake8-compliant formatting
Cleaning up docstrings
docstring changes to correctly format input/output, clarifying XYZ and reference white
Adding note for Luv colorspace to top docstring
Make six an external dependency.
Add python-six as Travis dependency.
Install six from PyPi.  Travis version is too old.
Format color conversion docstrings.
Merge pull request #1 from stefanv/luv-colorspace

Format color conversion docstrings.
Modify mpl_image_to_rgba to allow RGB input image

mpl_image_to_rgba produces a image of shape (M, N, 3, 4) when the input
image is already RGB. This is understandably confusing for downstream
processes, and this commit fixes it.
Fix ColorHistogram to work with Open File
Merge pull request #802 from jni/allow-rgb-save

Modify mpl_image_to_rgba to allow RGB input image
Bug: Fix label sequence in regionprops.

Closes gh-806.
Merge pull request #807 from josteinbf/regionprops-label-sequence

Bug: Fix label sequence in regionprops.
Add rank filter tests as data dir
Fix 16bit rank filter test in python 3.x