Support for multiparameter attributes

Multiparameter attributes, like those used with datetime_select and
time_select are understood, and the parenthesized portion is stripped
when checking permissions.

For example:
  @ability.can :update, :article, :date
will allow
  @ability.can? :update, :article, 'date(1i)'
  @ability.can? :update, :article, 'date(2i)'
  @ability.can? :update, :article, 'date(3i)'