Ongoing development to create abstracted Payment Gateway class with
DataCash-specific subclass

1. Parameter tags defined as class constants to assist implementing
program
2. Extra error-messages for exception-handling
3. 3D-Secure parameter values defined as constants
4. Assigning parameter array to $this->_arrParam and using directly
instead of many individual properties
5. Divided param authentication into themed methods
6. Added DataCash TxnDetails, Card and 3DSecure blocks