Add topic guide to scheduling tasks with Celery.

Branch: celery-doc
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Move celery doc out of applications; add lots of links

Branch: celery-doc
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Remove contrib.scheduler app

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Remove information about production deployment from celery topic.

We'll cover this when we document deployment. For now just leave
a link and a warning not to use the development configuration
for production.
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Add warning not to use Django broker in production

We'll flesh this out when we add documentation about deploying
in a later release, but for now we have to at least warn that
the configuration used in this tutorial must not be used in
production.
Updates in response to review
Branch: celery-doc
Merge pull request #298 from rapidsms/celery-doc

Add celery doc