Fix #123 There were two errors:

1. The script broke because the edx platform changed the default URL for
subtitles format. (before it was URL + /videoId=VIDEO_ID?language=LANG,
now it is URL + LANG?videoId=VIDEO_ID)
2. The script broke even if we didn't want the subtitles. I fixed this
adding the condition.

I also slightly refactored some variable names to make thinks clear. Now
It uses a less pythonic, but simpler style.