celery: simplify internal configuration and app creation

We used celery.app.app_or_default() which creates a "global fallback app
instance" which relies on the CELERY_LOADER environment variable to load the
configuration. That worked but was messy.

Instead, do something more like described in
http://docs.celeryproject.org/en/3.1/userguide/application.html where the app
is a celery.Celery() instance and configuration is loaded explicitly by its
config_from_object method. Using config_from_object we don't need explicit
invocation of import_default_modules and can take pass PylonsSettingsProxy
directly, leaving PylonsLoader unused and removed.

Modified by Mads Kiilerich.