Refactor Runner + Inspector bunch totally

* Split single Inspector class to subclasses FocusedInspector,
  KeepingInspector, SimpleInsector with base class Inspectors::Base.
  Instanciate inspector using Inspectors::Factory.

  That's allow us to move logic of 'focus_on_failed' and 'keep_failed'
  to FocusedInspector and KeepingInspector accordingly.

* Move all notification code to separate class (Guard::RSpec::Notifier).

* Move responsibility of reading ./tmp/rspec_guard_result to Runner.