0.8.3
* Fixed issue with CompanyCurrency object
* Added to_ref method to the Term object
* Added to_ref method to the Invoice object
* Fixed issues with RefundReceipt
* Added RefundReceiptCheckPayment object to RefundReceipt object
* Added from_json method to MetaData
* Added SendMixin to Estimate object
* Added SendMixin to PurchaseOrder object
* Added Memo field to PurchaseOrder
* Added support for TDS (special type of tax applied to Indian companies)