New beginner lessons on SQL

1.  Importing/updating material previously written for Software Carpentry instructors.
    *   Needs diagrams.
    *   Needs instructor notes.
2.  Created simple IPython Notebook magic to run queries and tablify output.
    *   Needs better error handling.

Once reviewers confirm content/level/pace:

1.  Delete `db.txt`.
2.  Decide whether to keep `survey.sql` in the repo for use by learners.