refactoring in number_to_rounded_converter

Refactored the long convert method in NumberToRoundedConverter class
@sgrif : I removed the attr_writer, as these variables are never written to from outside this class.
@rafaelfranca : I didn't keep them in the convert method, as it felt to me as some sort of a setup, before the actual convert is happening. Do you want me to move it back to convert ?