DOC: viewer: some corrections
MAINT: populate __init__.py
Add viewer to main docstring.
Add py3k compatibility for core widget and overlayplugin.

Dictionary keys are treated differently in py3k, causing the Canny
plugin to not work on py3k.
Merge pull request #891 from sciunto/tuto2

DOC: viewer: some corrections
Use the more idiomatic list().
Merge pull request #894 from blink1073/viewer_docstring

Add viewer submodule to skimage docstring (closes gh-878)
Merge pull request #895 from blink1073/fix-canny-py3k

Improve py3k compatibility for Viewer core widget and OverlayPlugin.