Add support for adding and updating roster items.
Roster items can be added or updated by sending a roster set request using the
new wokkel.xmppim.RosterClientProtocol.setItem. It takes a RosterItem as
the only argument. Note that changes in presence subscriptions for the contact
need to be done using presence as provided by
wokkel.xmppim.PresenceProtocol.
Author: ralphm.
Fixes: #56.