Allow for 'classic' gunicorn running

[kkaris]

This PR adds code that allows the gilda app to be run with gunicorn.

To run with gunicorn:

gunicorn -w 2 -b :8000 -t 600 gilda.app:gunicorn_app

To run as module:

python -m gilda.app --port 8000 --host 127.0.0.5

[bgyori]

I changed the implementation to remove the redundancy between init and main, and renamed the default app to gilda_app which doesn't conflict with the app submodule but isn't specific to gunicorn.