Fixed issue where HTML entities in URL were not escaped.

I may be wrong, but it seems HTML entities aren't escaped in URLs.

For example, for this link: https://www.youtube.com/watch?feature=player_embedded&v=DPAw3McLayc

RedditKit returns: https://www.youtube.com/watch?feature=player_embedded&amp;v=DPAw3McLayc

(Note the `&amp;` instead of `&`.)

I simply used the existing `NSString+HTML.h` class to unescape the HTML entities in `RKLink`. (Also, shouldn't it be `unescapedLink` not `escapedLink`?)