Features: control verboseness

This is not exactly a pull request, but a request for comments, due to the nature of the change.

The only change in behavior here is that I started using the python module `logging` to separate which messages are important from those that are just informative. This introduces some noise in the output of `coursera-dl`, and I don't know if it is desired.

From a coding point of view, it seems to me like a way forward (as we can control such verboseness with a command line option, like in the subsequent commits). On the other hand, I don't know if anybody is/was trying to parse the output of `coursera-dl` before (if they were, they would be living in a sin, already, as coursera itself changes the layout of the site and in one of the recent commits, @jplehmann inserted the name of the course being downloaded)...

So, before I committed this, I would like to receive some feedback, just to see if I am not doing anything (too) stupid.