Optionally create a single app instance per test case class

[jcomo]

Addresses #115

@acidjunk what do you think about this implementation? This flag can be set per TestCase class and otherwise changes nothing (same process for CI without having to run external servers).

[acidjunk]

Will test it this week in our testsuite. Codewise: LGTM, this will probably work just fine.

[jcomo]

@acidjunk did you get a chance to test this out to see if it works for you?

[acidjunk]

Tested it and it works like a Charm. I like the solution with the Mixin, feels Django-ish :)