changed breadcrumb helper to find models by their primary key instead of find_by_id

Issue 2238 - http://github.com/gregbell/active_admin/issues/2238

find_by_id was being used for all models and in the case of a model using a different primary key this fails.

activeadmin-0.6.0/lib/active_admin/view_helpers/breadcrumb_helper.rb:15
````
obj = config.resource_class.where(config.resource_class.primary_key => part).first if config
````

This as concise as I can come up with, however it still is a bit long..