Added 'flattened_attributes' method which allows pulling attribute values from objects associated with the serialized object without having to create a bunch of getter methods.

Example:

```
class UserSerializer < ActiveModel::Serializer
  flattened_attributes last_login: [:login_attempts, :last, :created_at],
                       favorite_food_name: [:favorite_food, :name],
                       favorite_food_description: [:favorite_food, :description],

end
```

As opposed to getter method style:

```
class UserSerializer < ActiveModel::Serializer
  attributes :last_login,
             :favorite_food_name,
             :favorite_food_description

  def last_login
    object.login_attempts.last.created_at
  end

  def favorite_food_name
    object.favorite_food.name
  end

  def favorite_food_description
    object.favorite_food.description
  end

end
```

Also adds the `attribute_method_mapping` method which allows you to determine which methods will be called on the object to get the value of an attributes. This is useful when you use `flattened_attributes` or you use different attribute names in your JSON than you do in your Rails models and need to know what these mappings are.
Added a test for for  method.