Moved logic from class_of_active_record_descendant(class) to the
base_class method. This method was confusing because it required
an argument, but that argument was 'self'.
Moved base_class tests to inheritance_test.rb and added some test
coverage for some untested cases.
The problem: We need to be able to specify configuration in a way that
can be inherited to models that include ActiveRecord::Model. So it is
no longer sufficient to put 'top level' config on ActiveRecord::Base,
but we do want configuration specified on ActiveRecord::Base and
descendants to continue to work.
So we need something like class_attribute that can be defined on a
module but that is inherited when ActiveRecord::Model is included.
The solution: added ActiveModel::Configuration module which provides a
config_attribute macro. It's a bit specific hence I am not putting this
in Active Support or making it a 'public API' at present.
