[nhl] Add an extractor for videocenter's categories (#1586)

It downloads the last 12 videos.

test/test_playlists.py

@@ -16,6 +16,7 @@
     UstreamChannelIE,
     SoundcloudUserIE,
     LivestreamIE,
+    NHLVideocenterIE,
 )
 from youtube_dl.utils import *
 
@@ -74,5 +75,14 @@ def test_livestream_event(self):
         self.assertEqual(result['title'], u'TEDCity2.0 (English)')
         self.assertTrue(len(result['entries']) >= 4)
 
+    def test_nhl_videocenter(self):
+        dl = FakeYDL()
+        ie = NHLVideocenterIE(dl)
+        result = ie.extract('http://video.canucks.nhl.com/videocenter/console?catid=999')
+        self.assertIsPlaylist(result)
+        self.assertEqual(result['id'], u'999')
+        self.assertEqual(result['title'], u'Highlights')
+        self.assertEqual(len(result['entries']), 12)
+
 if __name__ == '__main__':
     unittest.main()