Modify PostHandler.CreateSlug to allow non-ascii character

when i give a title with non-ascii characters, the posts slug will be empty. due to:
```title = Regex.Replace(title, @"([^0-9a-z-])", string.Empty);```

also i make calling of string.Equals with `StringComparison.OrdinalIgnoreCase` which makes the intent of the code more clearly, i hope.