Users+Books

This PR have:
1. User can get all that he have. Book can get all users that have this book.
2. If user add book will be created new book in database and will be add record in users_to_books table. If user just share his copy of alredy exists book will be add only record in users_to_books table.
3. User can delete jis copy of book from db — will be removed record from users_to_books table and will be checked if somebody anoter have copy of this book, If book don't have owners book will be removed from database.
4. If user trying add already exist book he will be inform about this and he will be have chance for adding his copy of book.
5. New buttons!

TODO:
- [x] make method getCurrentUser
- [x] find a way to fetch id from INSERT postgres with RETURNING attribute (Added in separete PR)
- [x] correction from comments

I need your opinion about page desing (screenshots 2-4)

SCREENSHOOTS

1. On default page you can found new button — info button.
![defaultpage](https://cloud.githubusercontent.com/assets/9048513/5046895/912d7e28-6c20-11e4-99e2-7f70a60ab526.jpg)

2. Info about book page if user don't login yet
How you can see button "Add book" vanished from header. Now you will see this button only if you login.
![unloginmoreinfo](https://cloud.githubusercontent.com/assets/9048513/5046913/d2180520-6c20-11e4-8c49-d9cb81a62f1c.jpg)


3. Info about book page then user logIn already but don't share his copy of this book(if he have one ofc)
![moreinfonotowner](https://cloud.githubusercontent.com/assets/9048513/5046935/17b7b3f0-6c21-11e4-9006-b99c5efb936f.jpg)

4. Info about book page then user login and he shared his copy of book.
![moreinfoowner](https://cloud.githubusercontent.com/assets/9048513/5046946/35910e12-6c21-11e4-87cf-b33c02ec12cd.jpg)

5. New button on user account page! Book info and book delete!
![newinaccountpage](https://cloud.githubusercontent.com/assets/9048513/5046958/4b091820-6c21-11e4-8662-de0294f6ff79.jpg)

let's rewrite ```Db::execute()``` for return ```statement``` instance