Fixed precision argument

Following up on Issue #2, here's a suggested fix. 

The test I did:

```javascript
console.log(Num('0', 2).toString()); // 0.00
console.log(Num('.0', 2).toString()); // 0.00
console.log(Num('.1', 2).toString()); // 0.10
console.log(Num('0.1', 2).toString()); // 0.10
console.log(Num('1.1', 2).toString()); // 1.10
console.log(Num('1', 2).toString()); // 1.00
console.log(Num('1.0', 2).toString()); // 1.00
console.log(Num('1.1111', 2).toString()); // 1.11
console.log(Num('1234', 2).toString()); // 1234.00
```