Support user items retrieval

Much like `<recent-items/>` except in our client we want a feed of subscribed channel items rather than an "**X per node**" feed.

We are currently using the recent items stanza at present with a large value of **max-items-per-node** which results in an inefficient slow query (for our use case).

This functionality adds <user-items/> which is much more efficient. Example:

```xml
<iq from='channeluser@example.com/ChannelCompatibleClient' 
         to='channelserver.example.com' 
         type='get' id='ri1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <user-items xmlns='http://buddycloud.org/v1'
                since='2012-12-04T23:36:51.123Z' />
  </pubsub>
</iq>
```