coverage is busted again, port the upcoming alembic approach over

[Mike Bayer]

at some point, when building the pytest suite the behavior that coverage started up before anything else went away. It seems that conftest importing sqlalchemy/testing is all that needs to happen to ruin it as apparently conftest is pulled in before coverage starts. I seriously thought I tested the crap out of this when i first did pytest but apparently I'm wrong.

upcoming in alembic is an old-is-new-again "bootstrap" solution, hacky like the old one but generalized a little more nicely to nose and pytest.

[Mike Bayer]

• remove some crufty old testing options
• reestablish the "bootstrap" system of loading the test runners in testing/plugin; using the updated approach we just came up with for alembic. Coverage should be fixed now when running either py.test or nose. fixes #3196
• upgrade tox.ini and start using a .coveragerc file

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