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
Only define MongoidPostDecorator when using Mongoid
Merge pull request #541 from haines/object_class_name_alias

Alias `object` to `#{object_class.name}`