Package updater task

[nicysneiros]

Issue: djangopackages#675

Description:

• Configures Celery in the project
• Creates a new task package.tasks.update_package_task to be called async from package_updater.py command

To Do:

• Transfer the logic to manage rate limit to the task
  • We should be able to retrieve the Github instance from Package.repo (which can return an instance of GitHubHandler) and then we can access ratelimit_remaining field in the task package.tasks.update_package_task
• Clean up rate limit management from other places