Add an extractor for tv.slashdot.org (closes #1192)

It uses the ooyala platform, so it just extracts the ooyala url.

youtube_dl/extractor/__init__.py

@@ -58,6 +58,7 @@ from .ringtv import RingTVIE
 from .roxwel import RoxwelIE
 from .rtlnow import RTLnowIE
 from .sina import SinaIE
+from .slashdot import SlashdotIE
 from .soundcloud import SoundcloudIE, SoundcloudSetIE
 from .spiegel import SpiegelIE
 from .stanfordoc import StanfordOpenClassroomIE