Add Amount.ToBTC convenience method.

Closes #26.

amount.go

@@ -89,6 +89,11 @@ func (a Amount) ToUnit(u AmountUnit) float64 {
 	return float64(a) / math.Pow10(int(u+8))
 }
 
+// ToBTC is the equivalent of calling ToUnit with AmountBTC.
+func (a Amount) ToBTC() float64 {
+	return a.ToUnit(AmountBTC)
+}
+
 // Format formats a monetary amount counted in bitcoin base units as a
 // string for a given unit.  The conversion will succeed for any unit,
 // however, known units will be formated with an appended label describing

amount_test.go

@@ -185,6 +185,13 @@ func TestAmountUnitConversions(t *testing.T) {
 			continue
 		}
 
+		// Verify that Amount.ToBTC works as advertised.
+		f1 := test.amount.ToUnit(AmountBTC)
+		f2 := test.amount.ToBTC()
+		if f1 != f2 {
+			t.Errorf("%v: ToBTC does not match ToUnit(AmountBTC): %v != %v", test.name, f1, f2)
+		}
+
 		// Verify that Amount.String works as advertised.
 		s1 := test.amount.Format(AmountBTC)
 		s2 := test.amount.String()