Add [NSFetchRequest sortByKey:ascending:selector:] method

I am proposing to add a [NSFetchRequest sortByKey:ascending:selector:] method,
which would make writing this kind of code more convenient:

NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES selector:@selector(localizedCompare:)];
request.sortDescriptors = @[descriptor];

new style:

[request sortByKey:@"name" ascending:YES selector:@selector(localizedCompare:)];