list properties/classes of strict vocabularies

I need to be able to access a list of allowable terms within a StrictVocabulary (I'm creating drop down menus).  This change is just a way to get the registered URIs in a list.

My use case really only requires this method on StrictVocabs, but I also considered moving @@properties up to Vocabulary so this could apply to registered terms in general. I'm open to making that change if it's deemed appropriate.