Added book ratings

![ratings](https://cloud.githubusercontent.com/assets/7677641/5070258/e6d792da-6e75-11e4-94cb-7f3713d1484b.jpg)

`rating` - readonly average value
We should not store `new rating` (weird name agree) in Book at all
just call `rateBook` method that process adding new rating to database - we have no needs to store this value at all