make InheritableHash recursive so we can store arbitrary data structures in it.
add new test for that data structure.
Config is now a InheritableHash itself which makes everything simpler. Config#add and #get instead of #[] and #[]=!
add Cloneable and more tests for inheriting.
fix nested InheritableHashes with Cloneables in it.
representable_attrs' inherit tests in config/inherit_test.rb now.
also assert that definitions get cloned properly.
move InheritableHash into Inheritable::Hash.
move Inheritable into inheritable.rb.
remove Config#_features.
cleanup.
document Config api.