Allow custom association name

Basically this:

    School.belongs_to_active_hash :status, :class_name => 'SchoolStatus' 

was failing because by default it tries to constantinize `'status'`.

One thing to note: at first I tried the patch without preserving a provided `:primary_key` to see how much breakage caused, and none of the tests failed... at least some tests should have, am I right?