Add features; fix bug; make other changes

In detail:

  * Fix bug: ignore an empty worksheet rather than trying to
    create a table for it.

  * Die early if the database already contains tables that
    will cause a name clash.  (This means we no longer annoy the user
    by making him wait while we create tables, only to crash when we
    reach a table whose name is already in use.)

  * Create unique indices for all possible columns.

  * Document these changes.

  * Change the instructions for installation.

  * Make a very minor internal improvement.