Add ability to cast int64's to strings

Resolves #18

cast_test.go

@@ -34,6 +34,7 @@ func TestToFloat64(t *testing.T) {
 func TestToString(t *testing.T) {
 	var foo interface{} = "one more time"
 	assert.Equal(t, ToString(8), "8")
+	assert.Equal(t, ToString(int64(16)), "16")
 	assert.Equal(t, ToString(8.12), "8.12")
 	assert.Equal(t, ToString([]byte("one time")), "one time")
 	assert.Equal(t, ToString(template.HTML("one time")), "one time")

caste.go

@@ -198,6 +198,8 @@ func ToStringE(i interface{}) (string, error) {
 		return strconv.FormatBool(s), nil
 	case float64:
 		return strconv.FormatFloat(i.(float64), 'f', -1, 64), nil
+	case int64:
+		return strconv.FormatInt(i.(int64), 10), nil
 	case int:
 		return strconv.FormatInt(int64(i.(int)), 10), nil
 	case []byte: