Ability to disable add/remove buttons

I had to disable "add" or "remove" buttons, to I tried this configuration:

    ->add('items', 'collection', array(
        'allow_add' => true,
        'widget_add_btn' => null,
    ));

but the button still appeared.

With this modification, add_button and remove_button are disablable
Merge pull request #854 from AppVentus/master

Ability to disable add/remove buttons