Add WeiboIE (closes #1039)

It just embed video from other sites.
Modified the _VALID_URL of Youku to catch embed urls.

youtube_dl/extractor/__init__.py

@@ -73,6 +73,7 @@ from .vimeo import VimeoIE
 from .vine import VineIE
 from .c56 import C56IE
 from .wat import WatIE
+from .weibo import WeiboIE
 from .wimp import WimpIE
 from .worldstarhiphop import WorldStarHipHopIE
 from .xhamster import XHamsterIE