Add primary_key option to :embed_in table

This patch allows the embed table to reference a different parent column as primary_key, so relation like these can also be translated: 

```
class Post < ActiveRecord::Base
  has_many :comments, :primary_key => 'unique_title'
end

class Comment < ActiveRecord::Base
  belongs_to :posts, :primary_key => 'unique_title'
end
```