Added support for courses in Preview mode that have been taught previously and are about to reopen

Hi, although potentialy useful to others, the code is not really tested (it worked for this class: https://class.coursera.org/nlp/lecture/preview), so feel free to test and edit. 

It could use some improvement in get_video(url):446, as it should raise an exception :'raise ClassNotFound("Missing/hidden videos?")' if no video is found even here.