Added methods to re-initialize and destroy sortable

Moved attached initialization of sortable into a method and run that method on attached.
Did the same for detached and created a destroy method.

Sortable.html

@@ -61,9 +61,10 @@
     },
 
     detached: function() {
-      this.sortable.destroy()
+      this.destroy()
     },
 
+
     initialize: function() {
 
       var templates = this.querySelectorAll("template[is='dom-repeat']")
@@ -122,6 +123,12 @@
 
     },
 
+    destroy: function() {
+      if(this.sortable) {
+        this.sortable.destroy()
+      }
+    },
+
     groupChanged             : function(value) { this.sortable && this.sortable.option("group", value) },
     sortChanged              : function(value) { this.sortable && this.sortable.option("sort", value) },
     disabledChanged          : function(value) { this.sortable && this.sortable.option("disabled", value) },