Execute dispatcher with Flask application context

This change provide a Flask application context in my Spyne service (`current_user`, `current_app`, Flask-SQLAlchemy `db`, config dict...). It is a really useful improvement.

It is a bit controversial to me. The `init_app` must not bind the app to the extension, but it already indirectly happens with `real_wsgi_app`, so it is not a real issue. The access to application context adds a lot of value to this extension.