Changed to allow interstitial page or not

Thanks for maintaining this project; it's really useful!

I'm not seeing the interstitial page from the last update (I'm inside the Stanford network; maybe that's the difference?) So I changed it to try to handle that page, and just continue if not.
Also, in Ubuntu with the newest version of BeautifulSoup, I had to do
from bs4 import BeautifulSoup
instead of 
from BeautifulSoup import BeautifulSoup
You may want to verify that that works for you also.

Finally, the code to skip previously-downloaded videos was only skipping if the mp4 was present. I changed it to also skip if the wmv was present.