XHamsterIE: fix video extension and add video description

1. Some videos have a wrong video extension e.g.:
$ youtube-dl -o "%(ext)s" --get-filename "http://xhamster.com/movies/1722902/.html"
flvcdn_hash=e5678088dd31481c86f76d70391520ef&cdn_creation_time=1376033082&cdn_ttl=14400

2. Some vieos have an description e.g.:
http://xhamster.com/movies/1503398/.html