Notify in the inspector when a service is dying.

QA:
Test the functionality in a sandbox GUI instance:
  - `make devel`;
  - deploy a service;
  - open the inspector for that service;
  - open the JavaScript console and run:
    `app.db.services.item(0).set('life', 'dying');`
  - you should see the "service is being destroyed" message
    in the inspector.

Test the functionality in a real Juju environment.
  - bootstrap an environment and deploy the gui (e.g. with juju-quickstart);
  - run `juju set juju-gui juju-gui-source="https://github.com/frankban/juju-gui.git dying-service"` 
    and wait some minutes for the new release to be built: 
    you can observe the progress with `juju debug-log` or running
    `tailf ~/.juju/local/log/unit-juju-gui-0.log` if you are using a local env;
  - deploy a service;
  - open the inspector for that service;
  - destroy the service, either by using the inspector "destroy" button or
    the cli command;
  - you should see the "service is being destroyed" message
    in the inspector.