Parser: accept empty rules (rules without properties)
Parser: also accept '-' in pseudo elements names
Eval: possible_values(): move building combinations to separate function
Eval Config Classes: Possible Values: function possible_values_all

- if possible_values_all exists, call this instead with the full parameter
  values sets
Eval::cond(): implement possible_values_all
Eval: Possible Values: use exceptions instead of hasattr()
Parse File: assign each statement an id
Get Where: move compile selector to own function
Stat: new function stat_filter_statements
Get Where: improved mapping back of set tags
Mapnik 2.0, expr-v2: change default placement to 'interior' (points, polygons)