Add placeholder color normalization

The browser-defined style for placeholder text color is overwritten by style for input elements in Firefox. Adding :-moz-placeholder style overrides that, bringing Firefox inline with other browsers.

@mathiasbynens made a test case for this @ https://tinker.io/be2f2

This change improves consistency of placeholder style between Chrome, Safari, and Firefox browsers.

normalize.css

@@ -468,6 +468,20 @@ input[type="search"]::-webkit-search-cancel-button {
     -webkit-appearance: none;
 }
 
+/*
+ * Improves consistency of placeholder style between Chrome, Safari, and Firefox browsers
+ */
+
+::-webkit-input-placeholder
+{
+    color: #AAA;
+}
+
+:-moz-placeholder
+{
+    color: #AAA;
+}
+
 /*
  * Removes inner padding and border in FF3+
  * www.sitepen.com/blog/2008/05/14/the-devils-in-the-details-fixing-dojos-toolbar-buttons/