[youtube:playlist] Login into youtube if requested (fixes #1757)

Allows to download private playlists

youtube_dl/extractor/youtube.py

@@ -1490,7 +1490,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor, SubtitlesInfoExtractor):
             })
         return results
 
-class YoutubePlaylistIE(InfoExtractor):
+class YoutubePlaylistIE(YoutubeBaseInfoExtractor):
     IE_DESC = u'YouTube.com playlists'
     _VALID_URL = r"""(?:
                         (?:https?://)?
@@ -1516,6 +1516,9 @@ class YoutubePlaylistIE(InfoExtractor):
         """Receives a URL and returns True if suitable for this IE."""
         return re.match(cls._VALID_URL, url, re.VERBOSE) is not None
 
+    def _real_initialize(self):
+        self._login()
+
     def _real_extract(self, url):
         # Extract playlist id
         mobj = re.match(self._VALID_URL, url, re.VERBOSE)