Add to_param to make use with url helpers easier

Now, you can pass an instance of ShortenedUrl to the url/path helper:

```
shortened_url = ShortenedUrl.find_by_unique_key("foo")
shortener_translate_path(shortened_url)
```

instead of

```
shortened_url = ShortenedUrl.find_by_unique_key("foo")
shortener_translate_path(shortened_url.unique_key)
```