Delegate reflect_on_association to the model class to support simple_form's association input

This fixes #122

Since simple_form relies on `Model.reflect_on_association` to discover model associations when using the association input 
(see https://github.com/plataformatec/simple_form/blob/ed3de1c2b566a962697529e824953f6b280e2867/lib/simple_form/form_builder.rb#L550)
we need to delegate this method from the form class to the model class.

Thanks to @smartygus for spotting this and providing an initial solution for this.
Uhm, can't we just do `model_name.reflect_on_association` here?