Exposed the internal isWorking variable as an observable 'isActivating'

Useful when binding loading indicatators when views are changing, I use it like this:

    <div class="shell">
      <div class="active-view" data-bind="compose: activeView"></div>
      <div class="loader" data-bind="css: { active: !activeView() || activeView.isActivating }">
        <div class="content"><i class="icon-spinner icon-2x icon-spin"></i></div>
      </div>
    </div>