Change CreationTimestamp to a util.Time and set in each storage implementation

Because time.Time doesn't work correctly with our YAML package, it is necessary
to introduce a type, util.Time, which serializes correctly to JSON and YAML.

Eventually we would like timestamping to cut across storage implementations;
for now, we set it in each storage.

pkg/util/time.go

@@ -0,0 +1,97 @@
+package util
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// Time is a wrapper around time.Time which supports correct
+// marshaling to YAML and JSON.  Wrappers are provided for many
+// of the factory methods that the time package offers.
+type Time struct {
+	time.Time
+}
+
+// Date returns the Time corresponding to the supplied parameters
+// by wrapping time.Date.
+func Date(year int, month time.Month, day, hour, min, sec, nsec int, loc *time.Location) Time {
+	return Time{time.Date(year, month, day, hour, min, sec, nsec, loc)}
+}
+
+// Now returns the current local time.
+func Now() Time {
+	return Time{time.Now()}
+}
+
+// Unix returns the local time corresponding to the given Unix time
+// by wrapping time.Unix.
+func Unix(sec int64, nsec int64) Time {
+	return Time{time.Unix(sec, nsec)}
+}
+
+// Rfc3339Copy returns a copy of the Time at second-level precision.
+func (t Time) Rfc3339Copy() Time {
+	copied, _ := time.Parse(time.RFC3339, t.Format(time.RFC3339))
+	return Time{copied}
+}
+
+// UnmarshalJSON implements the json.Unmarshaller interface.
+func (t *Time) UnmarshalJSON(b []byte) error {
+	if len(b) == 4 && string(b) == "null" {
+		t.Time = time.Time{}
+		return nil
+	}
+
+	var str string
+	json.Unmarshal(b, &str)
+
+	pt, err := time.Parse(time.RFC3339, str)
+	if err != nil {
+		return err
+	}
+
+	t.Time = pt
+	return nil
+}
+
+// MarshalJSON implements the json.Marshaler interface.
+func (t Time) MarshalJSON() ([]byte, error) {
+	if t.IsZero() {
+		// Encode unset/nil objects as JSON's "null".
+		return []byte("null"), nil
+	}
+
+	return json.Marshal(t.Format(time.RFC3339))
+}
+
+// SetYAML implements the yaml.Setter interface.
+func (t *Time) SetYAML(tag string, value interface{}) bool {
+	if value == nil {
+		t.Time = time.Time{}
+		return true
+	}
+
+	str, ok := value.(string)
+	if !ok {
+		return false
+	}
+
+	pt, err := time.Parse(time.RFC3339, str)
+	if err != nil {
+		return false
+	}
+
+	t.Time = pt
+	return true
+}
+
+// GetYAML implements the yaml.Getter interface.
+func (t Time) GetYAML() (tag string, value interface{}) {
+	if t.IsZero() {
+		value = "null"
+		return
+	}
+
+	value = t.Format(time.RFC3339)
+	return tag, value
+}