Implementation of the Backup/Restore feature

Please review the [detailed documentation](https://drive.google.com/file/d/0B4Pa6xrlwhLzMzlDRWNlSzFoMkE/edit?usp=sharing) and [the forum thread](https://bitmessage.org/forum/index.php/topic,3294.msg6796.html#msg6796)  regarding this feature.