New feature: isEnabled

Hi, I added the isEnabled feature. I am not sure if there is a better way to access the isEnabled attribute on the sortable from the sortable-item-handle. Also in my opinion it has a smell that I need two watches (one in sortable and one in sortable-item-handle) but it don't know how to make it better. Let me know what you think. Looking forward to have the feature for ng-sortable.