Add binary (un)marshalling support

Implement encoding.BinaryMarshaler and Binary.Unmarshaller. Add tests
to verify.

decimal.go

@@ -548,6 +548,18 @@ func (d Decimal) MarshalText() (text []byte, err error) {
 	return []byte(d.String()), nil
 }
 
+// UnmarshalBinary implements the encoding.BinaryUnmarshaler interface. As a string representation
+// is already used when encoding to text, this method stores that string as []byte
+func (d *Decimal) UnmarshalBinary(data []byte) error {
+	return d.UnmarshalText(data)
+}
+
+// MarshalBinary implements the encoding.BinaryMarshaler interface. As a string representation
+// is already used when encoding to text, this method restores that []byte-encoded string.
+func (d Decimal) MarshalBinary() (data []byte, err error) {
+	return d.MarshalText()
+}
+
 // NOTE: buggy, unintuitive, and DEPRECATED! Use StringFixed instead.
 // StringScaled first scales the decimal then calls .String() on it.
 func (d Decimal) StringScaled(exp int32) string {

decimal_test.go

@@ -1,6 +1,8 @@
 package decimal
 
 import (
+	"bytes"
+	"encoding/gob"
 	"encoding/json"
 	"encoding/xml"
 	"math"
@@ -300,6 +302,59 @@ func TestBadXML(t *testing.T) {
 	}
 }
 
+func TestBinary(t *testing.T) {
+	for x, _ := range testTable {
+
+		// Create the decimal
+		d1 := NewFromFloat(x)
+
+		// Encode to binary
+		b, err := d1.MarshalBinary()
+		if err != nil {
+			t.Errorf("error marshalling %v to binary: %v", d1, err)
+		}
+
+		// Restore from binary
+		var d2 Decimal
+		err = (&d2).UnmarshalBinary(b)
+		if err != nil {
+			t.Errorf("error unmarshalling from binary: %v", err)
+		}
+
+		// The restored decimal should equal the original
+		if !d1.Equals(d2) {
+			t.Errorf("expected %v when restoring, got %v", d1, d2)
+		}
+	}
+}
+
+func TestGOB(t *testing.T) {
+	for x, _ := range testTable {
+
+		// Create the decimal
+		d1 := NewFromFloat(x)
+
+		// Encode using GOB
+		b := new(bytes.Buffer)
+		err := gob.NewEncoder(b).Encode(d1)
+		if err != nil {
+			t.Errorf("error GOB encoding %v to binary: %v", d1, err)
+		}
+
+		// Restore from GOB
+		var d2 Decimal
+		err = gob.NewDecoder(bytes.NewBuffer(b.Bytes())).Decode(&d2)
+		if err != nil {
+			t.Errorf("error GOB decoding: %v", err)
+		}
+
+		// The restored decimal should equal the original
+		if !d1.Equals(d2) {
+			t.Errorf("expected %v when restoring, got %v", d1, d2)
+		}
+	}
+}
+
 func TestDecimal_rescale(t *testing.T) {
 	type Inp struct {
 		int     int64