Perform conversions in celery workers

[abingham]

We need to get conversion processing out of requests handling, and celery seems like a good way to do this. It's not 100% trivial, and it complicates the deployment landscape a bit, but it's the right thing to do.

[abingham]

It seems likely that we'll need to somehow pass arguments to the workers, e.g. mongo server details, decktape locations, etc.

If we use command line arguments (as seems likely), check out this:

http://stackoverflow.com/questions/27070485/initializing-a-worker-with-arguments-using-celery