Sql tut review

This draft SQL tutorial is now ready for review.

The single file here should be totally self-contained. Working through the tutorial doesn't depend on any other files that have been created in earlier branches.

The links in the file (to a gist, and to the sqlite site) exist and are up-to-date.

The current plan is for a specific person to try out the tutorial. Any interest is welcome, but bear in mind that it might undergo significant changes, before investing time in editing it, etc.

Please get in touch with any questions at all.