Class for inheritable, role-based permissions system (Role Based Access
Control - RBAC).

Custom methods can be placed on role objects. Authorization can be
performed either by checking whether the role name matches the required
name, or by testing (via can) whether the role can perform the method
required.

Two role are specified by default. At the top, superusers can do anything
($superuser->can( $action ) always returns a coderef). At the bottom, the
base role can do nothing ($base->can( $action ) always returns undef).

All roles are automatically capable of authorizing actions named for the
singular and plural of the role name.