Fixed marker cache to handle asynchronous arrivals to disposed maps.
Because icon generation is now asynchronous, it's possible for an icon
to arrive after the map that wants it, and its cache, has been
disposed.  Which caused a write into a nonexistent cache.  Instead, we
now ignore the request.

Presidents exhibit was broken.  The auto-position option wasn't
working.  Investigation suggests it hasn't been working for a very
long time---there's a 2 year old bug in the code.  For now, I've
removed use of that option.
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
Merge pull request #66 from karger/2013-11-08_presidents

Presidents exhibit was broken.  The auto-position option wasn't working.