manageStamps when prepending; fixes desandro/masonry#403

Author: desandro

bower.json

@@ -1,6 +1,6 @@
 {
   "name": "outlayer",
-  "version": "1.1.7",
+  "version": "1.1.8",
   "description": "the brains and guts of a layout library",
   "main": [
     "item.js",

outlayer.js

@@ -1,5 +1,5 @@
 /*!
- * Outlayer v1.1.7
+ * Outlayer v1.1.8
  * the brains and guts of a layout library
  */
 
@@ -729,6 +729,7 @@ Outlayer.prototype.prepended = function( elems ) {
   this.items = items.concat( previousItems );
   // start new layout
   this._resetLayout();
+  this._manageStamps();
   // layout new stuff without transition
   this.layoutItems( items, true );
   this.reveal( items );