Corrected map autoposition functionality and use of max-auto-zoom param.
The idea behind autoposition is that exhibit should render the map at
a position and zoom that is suitable for the rendered items.  A max-auto-zoom
parameter was defined to limit how zoomed in this auto-positioning could
be.  However, it was defined as a *per-item* parameter: in other words,
you would specify a max-auto-zoom for each item by specifying an exhibit
expression defining it.  This seems like overkill; it's unlikely that an
author would go to the trouble of defining such a parameter per point.
Also, the code wasn't working.  I've replaced it with a global parameter,
and made it work.
Added proper handling of map autoposition and max-auto-zoom parameters
and demonstration of same in presidents exhibit