[UstreamIE] Add support for "/embed/recorded" style URLs (Fixes #2990)

This commit adds support for "embed/recorded" style URLs as found on http://inside.twit.tv/blog/2014/5/14/ustream-goes-behind-the-scenes-at-twit.html (See: #2990). 

Git is being odd so just in case the diff doesn't generate: 

```git
-    _VALID_URL = r'https?://www\.ustream\.tv/(?P<type>recorded|embed)/(?P<videoID>\d+)'
+    _VALID_URL = r'https?://www\.ustream\.tv/(?P<type>recorded|embed|embed/recorded)/(?P<videoID>\d+)'

     def _real_extract(self, url):
     m = re.match(self._VALID_URL, url)

+        video_id = m.group('videoID')
+
+        if m.group('type') == 'embed/recorded': # some sites use this embed format (see: http://github.com/rg3/youtube-dl/issues/2990)
+            video_id = m.group('videoID')
+            webpage = self._download_webpage(url, video_id, note="Downloading embedded Ustream page")
+            desktop_url = 'http://www.ustream.tv/recorded/' + video_id
+            return self.url_result(desktop_url, 'Ustream')