def pmt(options = {})
      future_value = options.fetch(:future_value, 0)
      type = options.fetch(:type, 0)
      ((@amount * interest(@monthly_rate, @duration) - future_value ) / ((1.0 + @monthly_rate * type) * fvifa(@monthly_rate, duration)))
    end
def pmt(options = {})
      future_value = options.fetch(:future_value, 0)
      type = options.fetch(:type, 0)
      ((@amount * interest(@monthly_rate, @duration) - future_value ) / ((1.0 + @monthly_rate * type) * fvifa(@monthly_rate, duration)))
    end
create a new Loan instance
 @return [Loan]
 @param [Numeric] decimal value of the interest rate
 @param [Integer] Duration of the loan period
 @param [Float] Loan amount
 @param [Float] structure fee - fee for the market in percentages
 @param [Float] currency protection - Protection for currency changes - usually 3%, default to 0%
 @example create a 10.5% Nominal rate
 @see http://en.wikipedia.org/wiki/Nominal_interest_rate
 @api public