provide relaxed test discovery for pytest

 This is the spiritual successor to https://pypi.python.org/pypi/spec,
 but is built for pytest instead of nosetests, and rethinks some
 aspects of the design (such as increased ability to opt-in to various
 behaviors.)
 .
 Has it ever felt strange to put tests in tests/,
 then name the files test_foo.py, name the test classes TestFoo,
 and finally name the test methods test_foo_bar?
 Especially when almost all of the
 code inside of tests/ is, well, tests?
 .
 This pytest plugin takes a page from the rest of Python, where you
 don't have to explicitly note public module/class members, but only
 need to hint as to which ones are private. By default, all files and
 objects pytest is told to scan will be considered tests; to mark
 something as not-a-test, simply prefix it with an underscore.