Add pypy3 to tox and travis
Document automatic usage of db mark with Django's TestCase

Closes https://github.com/pelme/pytest_django/pull/70.
Merge pull request #139 from blueyed/travis-add-pypy3

Add pypy3 to tests / travis
tox/travis: test with both pytest 2.5 and 2.6

Fixes https://github.com/pelme/pytest_django/issues/151