Some testing improvements

Still familiarizing myself with the code, making the test infrastructure work for me
Oh, maybe I see the misunderstanding here: I've implemented a fallback to use the system nosetests in case nose is not installed in the virtualenv. Because nosetests is not properly used in plaid (yet), I mistakenly assumed nose may not be installed in the virtualenv, but the system nose had to be used. I just rechecked, and it seems nose is already in requirements.txt. With this, I'm happy to remove the fallback, and just hardcode the use of the nose installation in the virtualenv. Is this what you are requesting?