Added a 'tlNav' service for managing navbar state

* Currently only controls "visibility" property of the `tlBranding`
  directive, but will be expanded to include other attributes soon.
  This service can be used in `resolve` methods of your (ui-router)
  routing configuration to set the navbar state & behavior. For
  example:

    tlNavbar.inconspicuous();
    tlNavbar.standardVisibility();

  Such as:

    ...,
    resolve: {
      navVisibility: function (tlNavbar) { return tlNavbar.inconspicuous(); }
Merge branch 'generalized_directive'