Complete ginkgo's separation from gevent, adding async modules for threading/eventlet, remove serve_forever

[inconshreveable]

This also merges in the eventlet branch.

I'm not entirely happy with the error message construction and enumeration of available async modules living in core.Service. Maybe we should throw a more specific error that we catch in the runner?

[progrium]

This all looks pretty great, but I didn't mean remove serve_forever, just the recursive part. It's sort of an expected idiom borrowed from gevent borrowed from Python stdlib servers.

[kyleconroy]

@progrium any update on this? Would love to see this merged in

[kennethreitz]

Hmmm