oc: Implement Sharing Message Object

It acts as a proxy of MAPIStoreMailMessage and it manages the properties
defined in [MS-OXSHARE] Section 2.2 by storing them in the mail message
as eXtensible MIME headers which starts with X-MS-Sharing.

OpenChange/MAPIStoreMailMessage.h

@@ -35,6 +35,7 @@
 {
   BOOL headerSetup;
   BOOL mailIsEvent;
+  BOOL mailIsSharingObject;
   NSString *mimeKey;
   NSString *headerCharset;
   NSString *headerEncoding;