Use NSOperationQueue to filter out duplicates.

I've created some convenience methods for showing notice views in a queue, one after the other. It's also possible to filter out duplicates. The `isEqual` method just checks if it has the same `title` and same `message`. There's also a new button in the example project called `Small Error (queues)`, which does filter duplicates to demonstrate the new feature. The example code looks like:

```objc
WBErrorNoticeView *notice = [WBErrorNoticeView errorNoticeInView:self.view title:@"Queued Network Error" message:@"Check your network connection."];
WBNoticeOperation *operation = [NSOperationQueue addNoticeView:notice filterDuplicates:YES];
__block WBNoticeOperation *weakOperation = operation;
operation.completionBlock = ^{
   NSLog(@"Queued notice operation dismissed! Interactively: %@", weakOperation.dismissedInteractively ? @"YES" : @"NO");
};
```