Use app context when performing job

[joostdevries]

Based on #26, fixes #10, related to #3

This PR should make sure we use the Flask app's app_context when performing a job. This allows you to access stuff like flask.url_for etc.

Changes:

• Register the app on module global _flask_app
• Use that variable instead of current_app so you can access the config without app context present.
• Subclass rq.Worker to use the app context in FlaskRQWorker.perform_job