Associative fields should figure out association options automatically

It should not be necessary to specify the options :foreign_key, :primary_key, and :class_name when defining an associative field. Administrate should be able to figure these out from the association.

We should add this automatic configuration, mark these options as deprecated, and ultimately remove them.

Already making some headway at https://github.com/thoughtbot/administrate/compare/master...pablobm:automatic-associations