move rule cache off style objects as styles should be immutable - amends 1c4b67ba98b12ace - refs #916
remove uneeded rule_cache property
Merge branch 'master' of github.com:mapnik/mapnik into fix-rule-cache
Merge branch 'master' of github.com:mapnik/mapnik into fix-rule-cache
Merge pull request #1723 from mapnik/fix-rule-cache

Fix rule cache (TODO: use move semantics for rule_cache)