Improve Rx safety with BehaviorRelay

Replace BehaviorSubject by BehaviorRelay. A BehaviorRelay cannot emit an error (it makes no sens to emit an error when emitting a new navigation state)

Add iOS 9.0 compatibility

This release sets the deployment target to iOS 9.0

Fix DemoStep filename

This release renames DemoWeft.swift in DemoStep.swift

First release since transfer from twittemb/Weavy

Fully functional Reactive Flow Coordinator.