Convert negatives to strings

[jasonharrison]

package main

import (
	"fmt"
	"github.com/zelcon5/cash"
)

func main() {
	var m *cash.Cash

	m = cash.NewUSD().SetCents(1200)
	fmt.Println(m.String())

	m = cash.NewUSD().SetCents(-1200)
	fmt.Println(m.String())
}

Currently prints:

$12.00
$,-12.00

Corrected with my pull request:

$12.00
($12.00)