time: use "2006-01-02 15:04:05.999999999 -0700 MST" as String format

This change shows all the information present
in the Time value (now including fractional seconds)
and also arranges the fields so that, within a single time zone,
string comparison and time comparison agree.

R=golang-dev, rogpeppe, r
CC=golang-dev
https://golang.org/cl/5654078

src/pkg/time/example_test.go

@@ -51,7 +51,7 @@ func ExampleMonth() {
 	}
 }
 
-// Go launched at Tue Nov 10 15:00:00 -0800 PST 2009
+// Go launched at 2009-11-10 15:00:00 -0800 PST
 func ExampleDate() {
 	t := time.Date(2009, time.November, 10, 23, 0, 0, 0, time.UTC)
 	fmt.Printf("Go launched at %s\n", t.Local())

src/pkg/time/format.go

@@ -344,9 +344,9 @@ func formatNano(nanosec, n int, trim bool) string {
 }
 
 // String returns the time formatted using the format string
-//	"Mon Jan _2 15:04:05 -0700 MST 2006"
+//	"2006-01-02 15:04:05.999999999 -0700 MST"
 func (t Time) String() string {
-	return t.Format("Mon Jan _2 15:04:05 -0700 MST 2006")
+	return t.Format("2006-01-02 15:04:05.999999999 -0700 MST")
 }
 
 type buffer []byte