Updated code to pass flake8 tests

Hey Matt, hope you're doing well :smile: 

I've just updated this codebase to pass flake8 tests (i.e. PEP8 and pyflakes).

You can test it by simply running the following commands in your virtualen:

``` bash
pip install flake8
flake8 setup.py flask_failsafe.py test/test_app.py
```

Cheers
Fotis