Feature: Health checks

@ssorallen Would you take a look at the taskDetailsComponent update?

I updated this page https://github.com/mesosphere/marathon/wiki/Test-App-Commands to contain the toggle server i build to be able to toggle the health of the end point of the health check.
This should check for null so `hasHealth` is always a boolean. Later on you double negate `hasHealth` to cast it to a boolean. Ensure it's a boolean here, and you can remove double negation elsehwere:

    var hasHealth =
      model.get("healthChecks") != null &&
      model.get("healthChecks").length != null;