Implementation
setAdjustmentPositive()
https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Sales/Model/Order.php#L3919-L3925
setAdjustmentNegative()
https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Sales/Model/Order.php#L3911-L3917
getAdjustmentPositive()
https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Sales/Model/Order.php#L2017-L2025
getAdjustmentNegative()
https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Sales/Model/Order.php#L2007-L2015