Start reboot, refactor Signal into HotSignal

Disable a bunch of files for now.
Reboot Producer/Consumer as ColdSignal/Subscriber
:fire: ObservableProperty (for now)
:fire: Promise (forever?????????)
Re-enable ObjectiveCBridging with the types so far
Rename back to ColdSignal.asDeferredRACSignal.
WIP rebuild Action—harder, better, faster, etc.
WIP rebuilding ObservableProperty
Break out a RemovalToken struct from Bag
Finish restoring ObservableProperty, document it
Merge branch 'group-source-by-language' into swift-api-redux

Conflicts:
	ReactiveCocoa.xcodeproj/project.pbxproj
	ReactiveCocoa/Swift/Consumer.swift
	ReactiveCocoa/Swift/Producer.swift
	ReactiveCocoa/Swift/Promise.swift
	ReactiveCocoa/Swift/Signal.swift
	ReactiveCocoa/Swift/SignalingProperty.swift
Remove HotSignals from ObservableProperty

It’s simpler to just vend one API, and easy enough to convert from cold
to hot anyways.
Fix type signatures and documentation in Action
Add HotSignal.buffer() to get a ColdSignal
Add HotSignal.replay()
Add ColdSignal.start() to get a HotSignal
Add ColdSignal.then()
Add ColdSignal.combineLatestWith
Minor tweaks to cold <> hot conversions
Add ColdSignal.first()