Symbols: Bugfix, use icons_dir to store generated symbol images
Stat::property_values() et.al.: add parameter warn_unresolvable
Mapnik: fix complaining unguessable values

- fixes #57
fix
Stat: bugfix, add all parameters to cache id
Stat: improve cache handling

- don't need eval_true and warn_unresolvable
Stat: Bugfix, pass additional parameters when generating properties
Symbols/Icons: Bugfix, avoid and warn for illegal values