Modernize Django transaction management for issue #226

[akuchling]

I wanted to try the FastFixtureTestRunner, but found it didn't work with Django 1.8. I looked at the transaction-management features in Django 1.5 vs. 1.8 and came up with this set of changes.

This patch works well enough that my test suite of 255 tests ran successfully using FastFixtureTestRunner, but my testing wasn't exhaustive: for example, I didn't check that any fixture changes are rolled back as expected.

[coveralls]

Coverage increased (+0.5%) to 68.845% when pulling 8e0b823 on akuchling:master into 3b9dad7 on django-nose:master.

[jwhitlock]

I'm inclined to merge this because it works for you, but there are still no tests in the django-nose test suite that exercise FastFixtureTestRunner, so I'm not sure if it even works in Django 1.9, 1.10, etc.

[jwhitlock]

Maybe the tests can be imported from #258.