Have an implicit instance for Equivalent classes, (A === B)

This makes it unnecessary to make NOOP pickers such as: 

  implicit def BSONValuePickler: BSONPickler[BSONValue] = new BSONPickler[BSONValue] {
    def pickle(doc: BSONValue): BSONValue = doc
    def unpickle(v: BSONValue, path: List[String]): String \/ BSONValue =
      typecheck[BSONValue](v, path)(identity)
  }