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
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
Add some PEP8
Merge pull request #971 from stefanv/bug/views_are_copies

Do not allow `view_as_*` to return copies (closes #866)