Refs interface on Object Client

Still have to write the docs, but:

```ruby
kv = app[:items][:some_key]
ref_ids = kv.refs.map {|r| r.ref }  # enumerates over refs, similar to other enumerators
ref = kv.refs[ref_ids.first] # retrieves single Ref, is a subclass of KeyValue
ref.archival? # true for Refs, false for KeyValues
ref.tombstone? # false for a singly-retrieved ref, but could be true for a ref from the list
kv.refs.with_values.offset(10).take(10) # and of course, the usual enumeration/request helpers
```

The only thing left I'm debating is perhaps a method on KeyValue, perhaps #restore or #clobber that will perform a regular PUT without a condition header. Thoughts?

Closes #60