YoutubePlaylistIE: break only if there's no entry field in the response

Otherwise the Favorite videos playlist cannot be downloaded complete.
Also break if it reach the maximum value of the start-index.
YoutubeIE: Move the code from _real_initialize to a base class

This allows to reuse the code in other IEs without having to overwrite some parts.
Add an extractor for downloading the Youtube favorite videos(closes #127)