Run cloudfront invalidations asynchronously [#10]

Fixes a problem where repeated uploads, or many resizes, would exceed
Cloudfront's simultaneous-invalidation limit (3 per distribution).
Previously, MaxInvalidationErrors were just ignored, leaving a stale
cache lying around.

As of this commit the README is a liar; you can no longer run this on
Heroku's free tier, since you need a worker dyno.

setup.cfg is necessary to direct nosetests to the right directory; a
couple implementation files need a celery_broker_url setting at module
load time, and nosetests was scooping them up before setUpPackage could
set the fallback default.

The Invalidate task could just be defined with a @task-decorated
function, but then I can't make that monkeypatching subclass in the
test.
Convert the README to markdown
Update the README to reflect the use of workers
Remove a line of debug output
 Merge branch 'invalidation-retry' into 'version-5'

Defer Cloudfront invalidations into a Celery worker