Fetch application description property and Fetch Item references

Adding property  `appDescription` in `PKTApp` class.
Adding `fetchItemReferencesWithID` method in `PKTItem` class.
You actually only need one level of mapping here, like:

    NSArray *items = nil;

    if (!error) {
      items = [response.body pkt_mappedArrayWithBlock:^id(NSDictionary *itemDict) {
          NSMutableDictionary *mutItemDict = [itemDict mutableCopy];
    
          // Inject the app ID as it is not included in the response
          mutItemDict[@"app"] = @{@"app_id" : @(appID)};
    
          return [[objectClass alloc] initWithDictionary:mutItemDict];
      }]];
    }

    if (completion) completion(items, error);