Show a hint on startup if the inbox is empty

Fixes #403

// FREEBIE

js/views/hint_view.js

@@ -0,0 +1,12 @@
+/*
+ * vim: ts=4:sw=4:expandtab
+ */
+(function () {
+    'use strict';
+    window.Whisper = window.Whisper || {};
+
+    Whisper.HintView = Whisper.View.extend({
+        className: 'firstRun hint',
+        templateName: 'hint',
+    });
+})();