v1.1.0: Add Readiness state

Changes:

• Add a new Ready() method to the main State interface. It returns a channel that is closed once all background jobs are ready. If the tree contains no states that support readiness, the channel is closed automatically after the tree has been traversed.
• Add a new WithReadiness state initializer. It returns a new State and a ReadinessTail interface.
  The ReadinessTail should remain owned by the background job associated with the created State, as it carries the readiness signal.
  
  ReadinessTail has a single method -Ok(). It signals that the background job is ready. If Ok() is never called, readiness will remain blocked for all parent states, and the channel returned by State.Ready() will never be closed.