Changed from `--print-src` to `--print-source` to be more expressive

Adds a pseudo-hash to the filename of downloaded lectures based on their
filename on the ETH CDN.

This fixes the issue of not downloading mutliple lectures if they share
a local filenname

Closes #3

Something changed something with the backend, so the "Referer" is now
required when trying to acquire a cookie for "ETH" protected videos

Prints the HTTP response code when requesting a cookie for accessing
videos secured with "ETH"

to become more OS agnostic

(to unify code appearence)