Add timesamps! class method.

This will add ```created_at``` and ```updated_at``` if they don't exists already and defaults to ```DateTime.now()```

    class User < Dolly::Base
      timestamps!
    end

    user= User.new
    user.created_at # DateTime.now()
    user.updated_at # DateTime.now()

Also adds setters and [] methods

     user['created_at'] # DateTime.now()
     user['created_at'] = DateTime.now()
     user.updated_at = DateTime.now()