Implement QueryFactory and make ctors package scope

[nealeu]

Current API needs tidying up to hide the implementations. The following should work instead:

AttributeMatchQuery<Item> query = QueryFactory.findMatchesOfType(Item.class).forSubject(subject).withMatchStyle("potentialDates").withMaxResults(10).build();

and

AttributeMatchQuery<Item> query = QueryFactory.findMatchesOfType(Item.class).forSubjectRef(refToExistingItem).withMatchStyle("potentialDates").withMaxResults(10).build();