utils: Hardcode use of 10gen for the moment.
utils: Add some debugging statements.
edx_dl: Use same message as in the non-interactive branch.

This way, we avoid conflicts, if we ever have the need to merge or rebase
the code.
edx_dl: Use new URLs for downloading from 10gen.

Since they changed their name, we have to modify to avoid a 403 error.
util: Try to grab both the CSRF token and the session cookie.
util: Warn when we didn't get all cookies.
edx_dl: Fix regular expression for skipping optional parts.

The videos at speeds other than 1.0 are apparently optional and, with the
previous regular expression, we were forcibly expecting them, which is
something that is not guaranteed to exist.