Add WeiboIE (closes #1039)

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

youtube_dl/extractor/weibo.py

@@ -0,0 +1,37 @@
+# coding: utf-8
+
+import re
+
+from .common import InfoExtractor
+
+class WeiboIE(InfoExtractor):
+    """
+    The videos in Weibo come from different sites, this IE just finds the link
+    to the external video and returns it.
+    """
+    _VALID_URL = r'https?://video\.weibo\.com/v/weishipin/t_(?P<id>.+?)\.htm'
+
+    _TEST = {
+        u'url': u'http://video.weibo.com/v/weishipin/t_zjUw2kZ.htm',
+        u'file': u'98322879.flv',
+        u'info_dict': {
+            u'title': u'魔声耳机最新广告“All Eyes On Us”',
+        },
+        u'note': u'Sina video',
+        u'params': {
+            u'skip_download': True,
+        },
+    }
+
+    # Additional example videos from different sites
+    # Youku: http://video.weibo.com/v/weishipin/t_zQGDWQ8.htm
+    # 56.com: http://video.weibo.com/v/weishipin/t_zQ44HxN.htm
+
+    def _real_extract(self, url):
+        mobj = re.match(self._VALID_URL, url, flags=re.VERBOSE)
+        video_id = mobj.group('id')
+        webpage = self._download_webpage(url, video_id)
+        player_url = self._search_regex(r'var defaultPlayer="(.+?)"', webpage,
+                                        u'player url')
+        return self.url_result(player_url)
+