Add and use a Gradebook contextmanager for DbApp and DbApp tests

[lgpage]

Should fix #631

cc @ellisonbg

[lgpage]

@jhamrick I started messing around with AppVeyor CI for nbgrader and quickly tested this here and the python tests passed, but if you could please confirm if the tests pass on jenkins or not.

[jhamrick]

Nice. This makes me think I really should have just made the Gradebook a context manager to begin with, i.e. so you could do with Gradebook("sqlite:///gradebook.db") as gb:. But that would be a fairly major change -- maybe something to do for 0.5.0 😉

I will test this out on Jenkins now.

[lgpage]

Yeah making Gradebook a context manager would be nice 😄

[jhamrick]

Ok, Jenkins finished. The tests are failing on 2.7 but I think that's an unrelated issue. Thanks for fixing this!