attribute serialization error catching

[pedohorse]

• [x] define allowed attributes better than just "well, json.dumps doesn't raise, so...."
• [x] search and verify for every single usage of json
• [x] reuse serialization from new node serializers
• [x] tests!
  • [x] check all serializeable objects
  • [x] check some common non-supported objects
  • [x] ensure implementations in lifeblood lifeblood_client lifeblood_connection are compatible