Implement support for roster versioning

Roster versioning is defined in RFC 6121, section 2.2.6; the protocol
was originally described in XEP-0237.

Fixes SMACK-399

source/org/jivesoftware/smack/Connection.java

@@ -211,6 +211,11 @@ public abstract class Connection {
      */
     private String serviceCapsNode;
 
+    /**
+     * Stores whether the server supports rosterVersioning
+     */
+    private boolean rosterVersioningSupported = false;
+
     protected XMPPInputOutputStream compressionHandler;
 
     /**
@@ -829,6 +834,22 @@ public abstract class Connection {
         return serviceCapsNode;
     }
 
+    /**
+     * Returns true if the server supports roster versioning as defined in XEP-0237.
+     *
+     * @return true if the server supports roster versioning
+     */
+    public boolean isRosterVersioningSupported() {
+        return rosterVersioningSupported;
+    }
+
+    /**
+     * Indicates that the server supports roster versioning as defined in XEP-0237.
+     */
+    protected void setRosterVersioningSupported() {
+        rosterVersioningSupported = true;
+    }
+
     /**
      * A wrapper class to associate a packet filter with a listener.
      */