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.
Fix ColorHistogram to work with Open File
Add rank filter tests as data dir
Fix 16bit rank filter test in python 3.x
Add plugin return values for ImageViewer

The model supported is that plugins can return an overlay, some data,
or both. Each plugin therefore returns an `(overlay, data)` tuple in
which each element can be `None`.

To allow return values, the plugin need only override the `output`
method defined in the base Plugin class.

See discussions here:
https://groups.google.com/d/msg/scikit-image/0nkJM-WguXA/iqogBABa748J
and here:
https://github.com/scikit-image/scikit-image/pull/805
Allow OverlayPlugin to return the current overlay
Merge pull request #809 from ahojnnes/rank-setup

Fix rank filter tests
Update pip requirements.