Destroy editor when scope is destroyed

[benfoster]

Currently the editor is not destroyed when the directive scope is destroyed. This is a problem for us as when switching between editors in our application, redactor just get's re-inited/rendered each time it is selected. Fix:

        scope.$on('$destroy', function () {
          if (angular.isDefined(editor)) {
            $_element.redactor('core.destroy');
          }
        });

[TylerGarlick]

Can you do a pull request for that?

[JeromeLam]

@TylerGarlick PR #37 does exactly that, although coded differently.