(feat) added custom fields support from Thunderbird's address book

SoObjects/Contacts/NGVCard+SOGo.h

@@ -1,6 +1,6 @@
 /* NGVCard+SOGo.h - this file is part of SOGo
  *
- * Copyright (C) 2009-2014 Inverse inc.
+ * Copyright (C) 2009-2017 Inverse inc.
  *
  * This file is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -45,6 +45,10 @@
 - (NSString *) pager;
 - (NSCalendarDate *) birthday;
 
+- (void) addElementWithTag: (NSString *) elementTag
+                    ofType: (NSString *) type
+                 withValue: (id) value;
+
 @end
 
 #endif /* NGVCARD_SOGO_H */