Fix for dynamic DOM id

Fixed an issue where the directive would not properly run when using the ng-attr-id attribute to dynamically bind an id.

Usage example :

//HTML
<zingchart ng-attr-id="{{myId}}" zc-json="myJson"></zingchart>

//Javascript inside your controller
  $scope.myId = "chart-1";
  $scope.zcJson = { ...};