Added ✓ to delivered messages

Looks like this:

![image](https://cloud.githubusercontent.com/assets/801452/5533180/62fe3c9e-8a08-11e4-9e03-d2edc6180ad4.png)

If you wanted to use an icon library instead of unicode, let me know. I also moved the .timestamp classes from index.css to manifest.scss because it seemed like a better fit.