4 space indenting.
Now works with redesigned Coursera.

This is a commit long overdue. Heavy refactoring and changes made.
Removed code as there appear to no hidden video links.

Even in the courses from University of Washington.
More refactoring. Removed unused functions.
Simpler implementation of native downloading.
Log the context instead of printing it.
Beautiful progress display for native downloading.
Add recommendation to use `wget` or `curl`.
Use default logging format and fix level setting.
Added example on using filters.
Updated readme and other meta stuff.