code: public PresenceSubscriber addBuddy(String uri, long timeout) {
      return addBuddy(uri, DEFAULT_SUBSCRIBE_DURATION, null, timeout);
    }
docs: This method is the same as addBuddy(uri, duration, eventId, timeout) except that the duration
  is defaulted to the default period defined in the event package RFC (3600 seconds) and no event
  "id" parameter will be included.
  
  @param uri the URI (ie, sip:bob@nist.gov) of the buddy to be added to the list.
  @param timeout The maximum amount of time to wait for a SUBSCRIBE response, in milliseconds.
  Use a value of 0 to wait indefinitely.
  @return PresenceSubscriber object representing the buddy if the operation is successful so far,
  null otherwise.