Add scripting API to gunicorn

This is a stub for discussing a possible scripting API for gunicorn.

It is useful to have it if you want to integrate gunicorn inside you app (probably similar to #625).

Right now I'm using it in the following way:

```python
from flask import Flask
from gunicorn.app.flask_wsgi import FlaskApplication as Gunicorn


app = Flask(__name__)
gunicorn = Gunicorn(app)

...

if __name__ == '__main__':
    args = docopt(__doc__)

    if args['development']:
        app.run(debug=True, host=host, port=port)
    elif args['production']:
        gunicorn.run(workers=workers, bind='%s:%s' % (host, port))
```

I named the class `FlaskApplication` because I assumed that this is most useful in Flask setting, but this is probably a bad assumption.