Implements steeves dynamic tasks as task.replace. Issue #817
add a groupmeta_collection option to save group results in a different collection than results and add a 'bson' fake kombu encoder to allow pymongo to serialize natively data in mongodb
ghettomq is discontinued
added groupmeta default
Clarify the prefetch multiplier

For years, I thought CELERYD_PREFETCH_MULTIPLIER turned off prefetching, and I had a lot of weird problems.  Today I finally figured out 1 means disabled, 0 means PREFETCH ALL THE THINGS.  This fixes the documentation bug.
Added PyPy 3 to tox.ini.
Added PyPy 3 to the build matrix.

Also, I removed the custom PPA since they are available by default.
Fixed copy/paste typos.
cert for test has expired
Worker: Changed loglevel for unrecoverable error to critical.
Improve rate limit accuracy
Tests passing
Fixes MongoDB result backend URL parsing problem.  Closes celery/kombu#375
Merge pull request #2176 from thedrow/patch-1

Added PyPy 3 to the build matrix
Update configuration.rst

I removed duplicates (BROKER_FAILOVER_STRATEGY)
Merge pull request #2187 from tomislater/patch-1

Update configuration.rst
Docs: Improvements to example configuration file /etc/default/celeryd.
Django: Child processes must call django.setup if on Windows (Issue #2126)
Cosmetics
AMQP result backend: Call on_interval every second (always) to propagate parent errors