Make six an external dependency.
Add python-six as Travis dependency.
Install six from PyPi.  Travis version is too old.
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.
Incorporate @tonysyu's suggestions re:docstrings
Merge pull request #801 from stefanv/six_external

Make six an external dependency.
Add make instruction for doctest
Let travis run doctests
Fix corner doc tests
Fix corner_subpix function for corners near border
Add example for corner_subpix function
Fix doctest of remove_small_objects
Fix watershed doctest
Fix hough_line_peaks doctest