Friends_ralation and books database added

###Create_Friends_relation_and_books_Database
I . Initialized data (Users, Books, relationships) is set in db\seeds.rb. 
II. app\models\friendship.rb and app\models\users_books.rb are created to model the relationship between users and between user and book
Note:
1. Please run 'rake db:seed' before starting the server to initialize the data (use the data in db\seeds.rb)
2. Please fill in the book imageLink and amazonLink in seeds.rb as initialized data
3. Not sure if Non-English data (e.g. the French author name of 'The Little Prince') may cause problems on front end
4. Sure that user_book_relation can be created from one side, but not sure that friendships are one-way creatable too