Thinner overview
Thinner overview

Makes the overview table friendlier to computers without monster
monitors, by reducing the width of the rows.  Specifically:

 * Show only commodities that are actually traded by at least one
   tracked building.
 * Use acronyms for building types.
 * Use shorter timestamp formats.

Fixes #11

BartKieviet/Bookkeeper,BartKieviet/Bookkeeper