Made hello runnable without gunicorn in debug mode

You can run it like this:

    $ python hello.py

[Flask’s debug mode](http://flask.pocoo.org/docs/quickstart/#debug-mode) outputs better error details than gunicorn, during dev + debug time. Also, auto-reloads the server when source files change!
Merge pull request #4 from migurski/patch-2

Made hello runnable without gunicorn in debug mode