Constructor
string $rapperCmd Optional path to the rapper command to use.:
object EasyRdf_Serialiser_Rapper
public function __construct($rapperCmd = 'rapper') {
  $result = exec("{$rapperCmd} --version 2>/dev/null", $output, $status);
  if ($status != 0) {
    throw new EasyRdf_Exception("Failed to execute the command '{$rapperCmd}': {$result}");
  }
  else {
    $this->rapperCmd = $rapperCmd;
  }
}