title: Class directive not working when using ng-class to load the directive
body: <p>I am trying to load a 'class' directive using ng-class. but my directive is never loaded when i do that. The directive is a multipurpose directive, and I don't want to create an isolated scope on this. it will be loaded only when required, based on ng-class conditions hence not using attribute or element directive. has anyone tried doing this and succeeded?</p>
  
  <p>this directive is called using <code>&lt;div ng-class="someClass {{tooltip: enabled}}"&gt;&lt;/div&gt;</code>
  here <code>enabled</code> is a scope variable.</p>
  
  <pre><code>app.directive('tooltip', ['$timeout', '$location', '$rootScope', function (timer, $location, $rootScope) {
      return {
          restrict: 'C',
          transclude: true,
          link: function (scope, element) {
              var printContent = function () {
                  /*  uses the content of .tooltip-content if it is a complex html tooltip, 
                      otherwise
                      you can use the title attribute for plaintext tooltips
                  */
                  var tooltipContent = $(element).find('.tooltip-content').html();
                  if (!tooltipContent) {
                      tooltipContent = $(element).attr('title');
                  }
                  $(element).tooltip({
                      content: tooltipContent,
                      items: "img, a, span, button, div",
                      tooltipClass: "tooltip",
                      position: { my: "left+30 top", at: "right top", collision: "flipfit" },
                      show: { effect: "fadeIn", duration: "fast" },
                      hide: { effect: "fadeOut", duration: "fast" },
                      open: function (event, ui) { $rootScope.tooltipElement = event.target; }
                  });
              };
              timer(printContent, 0);
          }
      };
  }]);
  </code></pre>