extractor: Set age limit on some adult-related extractors.

More age limit of videos for adult-related sites.

Note that, for redtube, I explicitly left the variable containing the age
limit, since the comment justifying the age limit is a good thing to have.

That being said, I included the age limit field on the test, to better
reflect what the information extractor does (even if it may not break the
automated tests).

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>

youtube_dl/extractor/redtube.py

@@ -10,7 +10,8 @@ class RedTubeIE(InfoExtractor):
         u'file': u'66418.mp4',
         u'md5': u'7b8c22b5e7098a3e1c09709df1126d2d',
         u'info_dict': {
-            u"title": u"Sucked on a toilet"
+            u"title": u"Sucked on a toilet",
+            u"age_limit": 18,
         }
     }