coursera: Fix PEP-8 compliancy.
coursera: Remove function that is already in credentials module.

The body of the functions are slightly different and I would expect users
that used a custom-placed netrc file to have things break, but if nobody
complains, then we will leave it as is.
README: Silently drop mention of BS3.
README: Make clearer that the versions of the dependencies is important.
coursera: Add a stop-gap, dirty solution to the parsing problem.

We will implement Jonas's ideas for more flexibility soon.
Supress HTTPError in grab_hidden_video_url

Coursera always generates the `data-modal-iframe` attribute, even if
there are no videos. Hence, if a lecture doesn't have an mp4, the url
is invalid and the `grab_hidden_video_url` throws an 404 HTTPError.