Support empty title.

I'd like to support notice views without a title. Unfortunately I could not use `title:nil`, because this automatically uses the default title `Unknown Error` then. So for now I'm using an empty title to not change the behaviour and avoid breaking existing code:

```objc
[WBErrorNoticeView errorNoticeInView:self.view title:@"" message:@"Check your network connection."];
```

If you think it's okay to break it in a newer version (e.g. `3.1.x`), I'd be happy to make another pull request.