Fixes for coding style and maintainabity

Hi.

Here are some fixes for making it easier to keep coursera-dl working. There is more stuff that I plan on sending, but I lack time today.

BTW, since you already tried to modularize the program, I will break down some things to make the code easier to understand (or, at least, I hope so).

One of the first candidates is the downloading engine, which already merits a class to abstract what downloader is being used... I hope to have time to tackle this.

Regards.