Add iron-icons bower package. 🔨

Since its used in the demo. It was previously installed along with
paper-elements, but we now install paper elements separately.
Installing the addon did not result in an error, but building it
actually did.

bower.json

@@ -6,6 +6,7 @@
     "ember-qunit-notifications": "0.1.0",
     "polymer": "^1.6.1",
     "webcomponentsjs": "^0.7.22",
-    "paper-elements": "PolymerElements/paper-elements#^1.0.7"
+    "paper-elements": "PolymerElements/paper-elements#^1.0.7",
+    "iron-icons": "PolymerElements/iron-icons#^1.1.3"
   }
 }