Adding serializer adapter for #13

  * Adding configuration to allow for user-configured serializer
  * Created SerializerAdapterInterface
  * Created concrete serializers for JMS, json_encode based on above interface. JMS is used by default to support backwards-compatibility
  * Added toArray interface to response to support json_encode adapter (if 5.4 support is added, change over to JsonSerializable interface)
  * Updated README to document new features/functionality