Support ActiveRecord single table inheritance for decorators

The idea is that for a given base `Book` model, with a `BookDecorator` defined, if I create a `Novel` model extending `Book` and don't create a `NovelDecorator`, `BookDecorator` is retrieved by `ActiveDecorator` to decorate my `Novel` instance. Now, if you declare a `NovelDecorator`, it just works as usual, and does not automatically inherit from `BookDecorator`, which is what we want I think.

As I can see, pull requests are not merged since a long time, but I hope you'll have some time to check them out soon, there are many useful features in those PRs !