Move SQL reference material to end of lesson

The SQL reference material added in 56221b3c7bb is useful, but
shouldn't be a focus when delivering this lesson in workshops
since we are assuming complete novices and presenting this level
of detail up front would be too much.

This change moves the material to the end of the lesson and
provides hyperlinks to it in the main material to highlight its
existence and make it easy to jump to when viewing the material
online.
Merge pull request #67 from ethanwhite/move-sql-ref-mat

Move SQL reference material to end of lesson.