Handle test warnings

Addresses #1182.  Adds a `test_setup` function in `skimage._shared.testing` to set up the test environment which is called from each test folder in its `__init__.py`.  Uses the `skimage._shared.utils.all_warnings` context manager for known warnings, with comments about what is being suppressed.
Warnings are not elevated for the minimum Travis build (2.7), as it was kicking out over 40 warnings that were purely due to older versions of `numpy` and `scipy`, and addressing them would have added a lot of noise and skipped doc tests.  We check for this by looking for `TRAVIS_PYTHON_VERSION == 2.7`, to ensure we can still elevate warnings on local Py2.7 builds.
And would not `with warnings.catch_warnings()` work? I think it reads better.

Final thought: I don't know whether it's feasible but it would be nice to specifically discard *expected* warnings, not all warnings... Sorry for putting that out there... =P Thoughts?