PHP-1031: Implement MongoWriteBatch method to return current batch size

Added MongoWriteBatch->getItemCount() returning how many items are total
Added MongoWriteBatch->getBatchInfo() retrieving how many items are
in individual batch and its size (in bytes)
Don't throw exceptions on argument failures, as per cr feedback
Merge pull request #640 from bjori/PHP-1031-write-batch-metadata

PHP-1031: Implement MongoWriteBatch method to return current batch size

* bjori/PHP-1031-write-batch-metadata:
  Don't throw exceptions on argument failures, as per cr feedback
  PHP-1031: Implement MongoWriteBatch method to return current batch size