Implement Avg please...

This seems like this would be an easy implementation. Something really simple like:

```go
func Avg(numbers ...Decimal) (Decimal, error) {
    start := Zero
    for x := 0; x < len(numbers); x++ {
        start = start.Add(numbers[x])
    }
    return start.Div(MustFromFloat64(float64(len(numbers))))
}
```