Update ggplot lesson (004) with new violin behavior

[jcoliver]

Previous versions of ggplot2 would omit categories with < 3 observations from plot and the lesson has material based on that behavior (see Oceania). It no longer has this behavior, so lesson should be updated.

Would also be worthwhile to:

• Update variable names with _ in place of .
• Add introductory section for creating a new project.