Marshal base 60 floats quoted for YAML 1.1 compat.

Closes #34.

decode_test.go

diff --git a/decode_test.go b/decode_test.go
index a290645..46c94d4 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -377,6 +377,12 @@
 		"a: <foo>",
 		map[string]string{"a": "<foo>"},
 	},
+
+	// Base 60 floats are obsolete and unsupported.
+	{
+		"a: 1:1\n",
+		map[string]string{"a": "1:1"},
+	},
 }
 
 type inlineB struct {

encode.go

diff --git a/encode.go b/encode.go
index 1d928b0..9f0c699 100644
--- a/encode.go
+++ b/encode.go
@@ -2,8 +2,10 @@
 
 import (
 	"reflect"
+	"regexp"
 	"sort"
 	"strconv"
+	"strings"
 	"time"
 )
 
@@ -167,10 +169,32 @@
 	e.emit()
 }
 
+// isBase60 returns whether s is in base 60 notation as defined in YAML 1.1.
+//
+// The base 60 float notation in YAML 1.1 is a terrible idea and is unsupported
+// in YAML 1.2 and by this package, but these should be marshalled quoted for
+// the time being for compatibility with other parsers.
+func isBase60(s string) (result bool) {
+	// Fast path.
+	if s == "" {
+		return false
+	}
+	c := s[0]
+	if !(c == '+' || c == '-' || c >= '0' && c <= '9') || strings.IndexByte(s, ':') < 0 {
+		return false
+	}
+	// Do the full match.
+	return base64re.MatchString(s)
+}
+
+// From http://yaml.org/type/float.html, except the regular expression there
+// is bogus. In practice parsers do not enforce the "\.[0-9_]*" suffix.
+var base64re = regexp.MustCompile(`^[-+]?[0-9][0-9_]*(?::[0-5]?[0-9])+(?:\.[0-9_]*)?$`)
+
 func (e *encoder) stringv(tag string, in reflect.Value) {
 	var style yaml_scalar_style_t
 	s := in.String()
-	if rtag, _ := resolve("", s); rtag != "!!str" {
+	if rtag, _ := resolve("", s); rtag != "!!str" || isBase60(s) {
 		style = yaml_DOUBLE_QUOTED_SCALAR_STYLE
 	} else {
 		style = yaml_PLAIN_SCALAR_STYLE

encode_test.go

diff --git a/encode_test.go b/encode_test.go
index 83dc490..b9319db 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -220,11 +220,18 @@
 		"a: 3s\n",
 	},
 
-	// Issue #24. 
+	// Issue #24: bug in map merging logic.
 	{
 		map[string]string{"a": "<foo>"},
 		"a: <foo>\n",
 	},
+
+	// Issue #34: marshal unsupported base 60 floats quoted for compatibility
+	// with old YAML 1.1 parsers.
+	{
+		map[string]string{"a": "1:1"},
+		"a: \"1:1\"\n",
+	},
 }
 
 func (s *S) TestMarshal(c *C) {