Improve godocs. Add testable example for NewAmount

This is consistent with the rest of the code base.
i.e.: base58/bloom/hash160/hdkeychain.

example_test.go

@@ -0,0 +1,43 @@
+package btcutil_test
+
+import (
+	"fmt"
+	"math"
+
+	. "github.com/btcsuite/btcutil"
+)
+
+func ExampleNewAmount() {
+	amountOne, err := NewAmount(1)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(amountOne) //Output 1
+
+	amountFraction, err := NewAmount(0.01234567)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(amountFraction) //Output 2
+
+	amountZero, err := NewAmount(0)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(amountZero) //Output 3
+
+	amountNaN, err := NewAmount(math.NaN())
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(amountNaN) //Output 4
+
+	// Output: 1 BTC
+	// 0.01234567 BTC
+	// 0 BTC
+	// invalid bitcoin amount
+}