Issue title: MailCore2 ios: How to run MCOIMAPIdleOperation?
 Issue body: Hello,

I have an issue with MCOIMAPIdleOperation. See the following code snippet:

```
 MCOIMAPIdleOperation *idleOperation = [session idleOperationWithFolder:folder lastKnownUID:msgOb.msgUID];
    [idleOperation start:[self idleHandler:msgOb.msgUID]];



- (void (^)(NSError * error))idleHandler:(int64_t)_msgUID {

NSString *folder = @"INBOX";
void(^idleHandler)(NSError *error) = ^(NSError *error) {
    if (!error) {
        int64_t start = _msgUID + 1;
        int64_t end = UINT64_MAX;

        MCOIMAPMessagesRequestKind requestKind = MCOIMAPMessagesRequestKindHeaders;
        MCOIndexSet *uids = [MCOIndexSet indexSetWithRange:MCORangeMake(start, end)];

        MCOIMAPFetchMessagesOperation *fetchOperation = [session fetchMessagesByUIDOperationWithFolder:folder
                                                                                                requestKind:requestKind
                                                                                                       uids:uids];

        void(^fetchHandler)(NSError*,NSArray*,MCOIndexSet*) = ^(NSError *error, NSArray *fetchedMessages, MCOIndexSet *vanishedMessages) {
            if(error) {
                NSLog(@"Error downloading message headers:%@", error);
            }

            // Iterate through the messages...
            for (MCOIMAPMessage *message in fetchedMessages) {
                NSLog(@"downloaded message with ID: %i", message.uid);
            }
        };

        [fetchOperation start:fetchHandler];
    } else {
        NSLog(@"There was a problem with the idle connection!");
    }
};

return idleHandler;

}
```

when i call this, the application gets crashed.
Please guide me, how can i notified for new incoming mail.

Please Help!

 Comments: 
 Comment 0: HI Pankaj,

Have you managed to get the notification on new incoming mail ?

Please suggest me possible ways of doing it.

Thank you.
Sandip.

 Comment 1: Please open another issue if you're having trouble with something other than the sandovalchristopher@example.com.  If you're seeing crashes, then post a crash log.  Otherwise, the code he's got (with a few modifications) is sufficient to get an IDLE request running.

{'source': 'GithubIssues', 'repo_name_with_owner': 'MailCore/mailcore2', 'issue_id': 20611004, 'repo_stars': 2447.0}