Fixed: removed discussion on k-nearest neighbor exclusion models which do
not cover the lattice perfectly.
Fixed: extra assumption: particle configurations are determined by their
defects.
Fixed: miscellaneous minor tweaks, formatting and typos.
Added: discussion about Lee-Yang zeros and their relation to the
high-fugacity expansion.
Added: open problem: soft exclusion potentials.
Added: extended bibliography.
Changed: Particles in figures are now always centered on the lattice, not
the dual lattice.
Changed: improved Makefile.
