Fix: Use a reflection cache in BeanCopier

+ Boosts performance by 99.8% in a simple benchmark that copies 500
  stories from Story -> StoryTO. Tests were done with a proper
  microbenchmarking tool (Google Caliper).

benchmark     us linear runtime
 Original 379147 ==============================
      New    738 =

+ Reflection caches don't play well with JRebel, but bean property changes
  in affected types are not too common, so this shouldn't be an issue