Use AM::Serializers 0.7 on Rails 3.0

Due to rails-api/active_model_serializers#304
Alias `object` to `#{model_name}`

If we have an `ArticleDecorator` there is going to be a method
added to it named `article` which is convenient for the user.
Require NameError#missing_name? from ActiveSupport
Improve aliasing to object class name