set_primary_key attribute not inherited

I'm expecting that if I change the primary key attribute on a base class, then subclasses will use the new primary key attribute. This does not appear to be the case.

``` rb
class M1 < Perry::Base
  attributes :foo
  set_primary_key :foo
end

M1.primary_key # => :foo

class M1 < M2; end

M2.primary_key # => :id
```
