Add Money#round method

I propose a new method on the Money class. The `Money#round` method is an explicit way to round the money value to the appropriate fractional for the associated currency. This method would be super helpful when working in infinite_precision mode and would enable business processes where rounding only occurs at specific places in the calculation. For example:

```ruby
Money.infinite_precision = true

m = Money.new(2.2) + Money.new(1.3) #=> Money.new(3.5)
m.round #=> Money.new(4)
```

It also allows the specification of a rounding mode
```ruby
Money.new(3.9).round(BigDecimal::ROUND_DOWN) #=> Money.new(3)
```