[youtube] Add an extractor for Youtube recommended videos (":ytrec" keyword) (closes #476)

The new extractor and YoutubeSubscriptionsIE are subclasses of YoutubeFeedsInfoExtractor, which allows to fetch videos from http://www.youtube.com/feed_ajax
ComedyCentralIE: support the extended interviews urls (fixes #1079)
YoutubeDL: support download of videos splitted in parts
TudouIE: implement the parts field in the info_dict
Add a post processor for joining video parts
Don't download all the video parts if the final video exists
Use a dictionary for each of the parts in the parts field of the info_dict

Some multipart videos may requires rtmpdump, they would need additional fields.
Check if the info_dict has an url or a parts field in the tests
Make the join videos class behave like a normal PostProcessor, executed if the "--join-parts" is given