Add support for CAS LogoutRequests

Fixes #2346
This changes the serviceURL sent by SOGo to the CAS server:
  /SOGo/so/ -> /SOGo/so/index

SoObjects/SOGo/SOGoCASSession.h

@@ -51,6 +51,8 @@
 + (SOGoCASSession *) CASSessionWithIdentifier: (NSString *) newIdentifier
                                 fromProxy: (BOOL) fromProxy;
 
++ (void) handleLogoutRequest: (NSString *) logoutRequest;
+
 - (NSString *) identifier;
 
 - (NSString *) ticket;
@@ -65,4 +67,13 @@
 
 @end
 
+@interface CASLogoutRequest : NSObject
+{
+  NSString *sessionIndex;
+}
+
+- (NSString *) sessionIndex;
+
+@end
+
 #endif /* SOGOCASSESSION_H */