[Fix #89] PostUpdateValidator service and specs.

Why a "standalone" service rather than a DSO? This should be usable for *either* a `Post` entity *or* a `PostData` instance, with either an accessor-method object or a Hash containing the data to be updated. Or at least that's been my justification up to now.

Code and specs written, but won't actually merge this until clients of the service are up and working.