Changed all instances of 'module' to 'lesson' in code, comments, directories, and file names

I know everyone is probably going to hate me for this, but if we're going to make the shift in terminology, we might as well get it over with now. Please feel free to test my changes heavily before we merge into master. Obviously, these changes will have to be merged into other branches as well (once we're comfortable with them.)

This should, however, work just fine, since I replaced every instance of 'module' with 'lesson' and every instance of 'Module' with 'Lesson" throughout the entire swirl package directory. I didn't mess with any functions/variables with the word 'mod' in them. You have to pick your battles....

__Note that this pull request incorporates Gina's commits, which gave existing lessons more descriptive names. Thanks, Gina!__