non-string dict keys in django-celery configs

This fix allows celery-flower to not have problems displaying
configurations for projects that still use configurations embedded
in django settings files.  In this instance there are some int
dict keys that are totally unrelated to celery but that are causing
frequent error messages in the celeryd logs.
Adds stats for eventlet pool
Fix handling of non-string keys in filter_hidden_settings
Merge branch '3.1' of github.com:celery/celery into 3.1
Updates system configuration
PromiseProxy: Only remove original object if evaluation succeeded
Multi: stopwait was not in usage
Fixes weird traceback issues with connection_or_acquire + producer_or_acquire
Make empty ResultSet support get()
Cosmetics for #2041
Tests passing
Fixes duplicate nodename warning.  Closes #2046
Worker --detach default workdir is now CWD
Worker --detach with C_FAKEFORK no longer closes open fds.  Closes #2044
Stupid zsh is messing with our beloved ksh heritage :chicken:

Closes #2038
Clarify behavior of rate limits in docs
signature(s, app=app) did not work properly when passing kwargs, as dicts were not properly upgraded to their subtask_type (e.g. Sig -> group)
Fix test for working_directory
Be more selective about how file descriptors get removed from Kombu's hub.

Given that file descriptor changes appear to get triggered on the Kombu side, these changes
may not make a material impact.  However, to make things more consistent with the
changes introduced in https://github.com/celery/kombu/pull/353, the changes have been
updated here.

This change would also help allow refactoring for remove_reader()/remove_writer() to
be smarter about how file descriptors get managed in the future (i.e. using a counter
instead of removes() to avoid possible race conditions with file descriptors being reused)
Fixes func -> fun (Issue #2032)