Pytest-cache

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- fix issue11: make tox.ini only pull in pytest<2.8 since pytest-2.8 integrates
pytest-cache functionality.

- use json for storing cache information. Thanks
Mike Amy for an initial related PR.
- no longer deselect all tests when running
a subset of all tests while other tests
are still remembered as failed

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- handle collect failures by considering all tests
below a failed collector as failed

- merged "--ff" (failedfirst) option to run all tests but
run the last-failed ones first. Thanks Jack Riches.

- fix issue6 - --clearcache doesn't bail out if .cache doesn't exist,
thanks longlho

- fix issue4: consider setup time skip of a previous failure as no
longer failing

- ensure --cache output is sorted

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- initial release