Create form element for roles

[MatthiasMolitor]

Create a form element that can be used to select a role.

The element should be able to build its selectable content from a Zend_Acl object.
It should be possible to add a filter which ensures that only assignable roles are displayed (based on ACL).
Another feature could be that the hierarchy of the roles is indicated.

[MatthiasMolitor]

The filter could be implemented via lambda function that indicates if an entry is accepted:

function ($role, Zend_Acl $acl) {
    // Decide if $role is available.
    return true;
};