Avoid initialization of Celery when used as a package

Previously, a cascade of imports caused a Celery app to be initialized whenever a shared task was imported. This is undesired: the application calling the shared task should be allowed to initialize and use its own Celery app. ECOM-2249.

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Enable authentication via JWT for ecommerce workers.

ECOM-2331

Configure Travis to deploy to PyPI when tags are pushed

Log warnings instead of errors when a task fails but can be retried

Prevent Celery from removing handlers on the root logger.

Allows setting custom logging handlers. Also fix service variant name.

fix

Add New Relic agent as an optional dependency

Asynchronous order fulfillment

Implements the ecommerce worker and a Celery task responsible for managing order fulfillment. ECOM-2242.