Fixed not being able to retrieve the permalink when it contains unicode characters

If you use RedditKit to retrieve a post containing unicode characters, such as http://www.reddit.com/r/RenameMe/comments/1gxyac/이것은_웹_페이지입니다, you won't be able to retrieve the permalink.
The call to URI.join() throws an InvalidURIError.

The fix is to encode the URL before passing it to URI.join().