Some additions to the user guide: getting started section +
section on common numpy operations.
Minor modifications following review by Johannes
Do not allow view_as_* to return copies (closes #866)
Take a softer approach and raise a warning when a non-contiguous input is provided
Update view_as_windows docstring
Fix typo in docstring
Update viewer example to use plugin outputs
Update lineprofile examples to use returned output
Update plugin class doc example to use output
DOC: Clarify UMFPACK warning; use if statements instead of assert
FIX: Shortcut output or catch trivial cases in random_walker

Also:
* New tests to cover these new checks
* Improvements to docstrings and user warnings
* Generalize handling of `sampling` in accordance with docstring
* Some extra whitespace to improve readability
TST: Add unit test for length-2 spacing with rank-2 input
Update Canny examples
Add all_warnings context manager

The context manager searches for __warningregistry__ entries that
Python leaves all over the place, clearing them, thereby ensuring
that warnings will always be raised.  This is necessary for the test
suite to detect warnings even if they were raised before under
the "once" filter.
Update shape tests to use all_warnings
In all_warnings context manager, return warnings for examination
Move all_warnings to skimage._shared.utils
Copy list of modules since it may change during iteration
Added Hubble Deep Field image.
Added example and pep8 changes.