Containers support

1. Support ActiveRecord as tag ("#{obj.class.name}-#{obj.to_param}")

2. Detect dependencies of fragments containing other fragments, e.g.,  several article fragments displayed in a section fragment, and when the inner fragment is expired also expire its containers.
    In the example, today changing one of the articles' title won't be reflected in the section, since it's cached.
    The feature allows tagging the article fragment with the article object, then on_save call Cashier.expire(self), which will also expire the section.