Parent readonly in multiselects

This adds a parent_readonly option to has_many associations which can be used to hide/show the editing and adding features of multiselects.

I needed this because my parent association was excluded from rails_admin's models which caused an error when someone clicked the "Add another" button or double clicked an item in the select box.

I am willing to give this some more love (testing + documentation) if someone confirms that it would be useful. Just wanted to make sure in advance as it is my first contribution to rails_admin.