First three lessons on SQL for beginners
More database lessons for novices.  To do:
*   `group by` in aggregation
*   data hygiene
*   explain the `%%sqlite` magic
1. Filled in 'group by'.
2. Put data hygiene in discussion of joins.
3. Got rid of 'next steps' sections.
Cleaning up before submitting pull request
Merge branch 'master' into new-sql-beginner-lessons
Merge pull request #135 from gvwilson/new-sql-beginner-lessons

New beginner lessons on SQL