coursera: Die as soon as we detect error in command line arguments.
Fix preview pages

Preview pages don't require authentication.
coursera: Simplify passing arguments to download_lectures.
Merge pull request #127 from jonasdt/preview

Don't use authentication for preview pages. Fixes #121.
coursera: Add some debugging messages for preview/regular mode.
coursera: Describe better the purpose of get_syllabus_url.
cosmetics: Remove camel case.
coursera: Remove mutable value from default value of parameter.
coursera: Remove unused modules.