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

test/views/last_seen_indicator_view_test.js

@@ -9,4 +9,11 @@ describe('LastSeenIndicatorView', function() {
         assert.equal(view.count, 10);
     });
 
+    it('increments count', function() {
+        var view = new Whisper.LastSeenIndicatorView({count: 4});
+        assert.equal(view.count, 4);
+        view.increment(3);
+        assert.equal(view.count, 7);
+    });
+
 });