Subscriptions with metadata

Users should be able to query for subscriptions and metadata in a single stanza, as in:

```xml
<iq from="juliet@capulet.lit/bc-app" to="buddycloud.capulet.lit" type="get" id="recent-items:1"> 
  <pubsub xmlns="http://jabber.org/protocol/pubsub"> 
    <subscriptions-with-metadata xmlns="http://buddycloud.org/v1"/> 
  </pubsub> 
</iq>
```