Make the help output more informative.
Partially revert previous commit to revert argument positions.

We want to make the username and the password be "optional", while making
the course ids to be mandatory.
Move getting the list of courses to a separate function.
Move code to download lectures to a separate function.
gitignore: Add list of files to ignore.
cleanup: Remove unused scripts.
Reorganize the tree to make the program import'able as a module.

We do this while still maintaining the ability to run the script as a
program.