Forward given 'start' argument.
Convert locals and magic numbers to instance variables.
Minor clean up of TransactionExporter.

* Remove unused 'export' method.
* Move retrieval of last start value to method.
Extract TransactionParser::Row type.

The knowledge of the structure of a transaction row and how to convert
it into an Order model has been pushed into an explicit class.

The TransactionParser now delegates this conversion to the Row.