Many changes; added Agents & Books tabs.

- Moved GUI, GUIGraphArea, GUIInfoWindow to gui::GUI, gui::GraphArea,
  gui::InfoWindow.
- Added Agents (i.e. readers) and Books tabs to main GUI window, which
  contain lists of current simulation agents and books.
- Added authored book list to author dialog window.
- new Book::marketPeriods() which returns the number of periods the
  book stayed on the market.
- new Book::copies() which queries the simulation's agents for the
  number of copies of this book in the simulation.
- ProfitStream overhaul: now constructs new profit stream beliefs for
  pre-defined ages, which start with (nearly) non-informative priors,
  thus no longer requiring the prior at construction time.
- Linear<K>: redocument/rename fields since the parameters are
  parameters, not priors: they are only used as priors when updating;
  otherwise they are simply distributional parameters.
boost::any isn't actually used/needed.
Docment book and reader TreeModel column objects.