Keep last seen indicator around for five seconds

Helps calm the user experience a little more, makes it easier to
understand what's happening when messages are coming in quickly.

FREEBIE

js/views/last_seen_indicator_view.js

@@ -5,12 +5,27 @@
     'use strict';
     window.Whisper = window.Whisper || {};
 
+    var FIVE_SECONDS = 5 * 1000;
+
     Whisper.LastSeenIndicatorView = Whisper.View.extend({
         className: 'last-seen-indicator-view',
         templateName: 'last-seen-indicator-view',
         initialize: function(options) {
           options = options || {};
           this.count = options.count || 0;
+          this.start = Date.now();
+        },
+
+        isOldEnough: function() {
+          var now = Date.now();
+          if (now - this.start > FIVE_SECONDS) {
+            return true;
+          }
+        },
+
+        increment: function(count) {
+            this.count += count;
+            this.render();
         },
 
         render_attributes: function() {