Refactored for Swift beta 4 compatibility
Merge branch 'swift-development' into pr/1422

Conflicts:
	ReactiveCocoa/Action.swift
	ReactiveCocoa/EventBuffer.swift
	ReactiveCocoa/ObjectiveCBridging.swift
	ReactiveCocoa/Signal.swift
Make Atomic and Bag internal-only
Make belated additions public

This stuff was added to RAC after the initial access control PR, so
they just weren’t around yet.
Make nested typealiases public
Make _empty consumer functions public

They’re used as default parameter values in Consumer.init, so I think
they need to be public.
Remove duplicative access control on extensions

It’s just shorthand for annotating each member. Let’s do that instead,
for consistency.
Make SignalingProperty.signal and .value public

This class is meant to be used and mutated by consumers, so it needs to
expose the machinery.
Make TestScheduler.ScheduledAction private
Remove prefix underscores on private functionality

Thanks, Real Access Control!®
Merge pull request #1424 from ReactiveCocoa/pr/1422

Refactored, harder, for beta 4 compatibility