Use rfc3339 timestamps in telegraf log output

closes #1564

also add unit and benchmark tests

logger/logger_test.go

@@ -0,0 +1,62 @@
+package logger
+
+import (
+	"bytes"
+	"io/ioutil"
+	"log"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestWriteLogToFile(t *testing.T) {
+	tmpfile, err := ioutil.TempFile("", "")
+	assert.NoError(t, err)
+	defer func() { os.Remove(tmpfile.Name()) }()
+
+	SetupLogging(false, false, tmpfile.Name())
+	log.Printf("I! TEST")
+	log.Printf("D! TEST") // <- should be ignored
+
+	f, err := ioutil.ReadFile(tmpfile.Name())
+	assert.NoError(t, err)
+	assert.Equal(t, f[19:], []byte("Z I! TEST\n"))
+}
+
+func TestDebugWriteLogToFile(t *testing.T) {
+	tmpfile, err := ioutil.TempFile("", "")
+	assert.NoError(t, err)
+	defer func() { os.Remove(tmpfile.Name()) }()
+
+	SetupLogging(true, false, tmpfile.Name())
+	log.Printf("D! TEST")
+
+	f, err := ioutil.ReadFile(tmpfile.Name())
+	assert.NoError(t, err)
+	assert.Equal(t, f[19:], []byte("Z D! TEST\n"))
+}
+
+func TestErrorWriteLogToFile(t *testing.T) {
+	tmpfile, err := ioutil.TempFile("", "")
+	assert.NoError(t, err)
+	defer func() { os.Remove(tmpfile.Name()) }()
+
+	SetupLogging(false, true, tmpfile.Name())
+	log.Printf("E! TEST")
+	log.Printf("I! TEST") // <- should be ignored
+
+	f, err := ioutil.ReadFile(tmpfile.Name())
+	assert.NoError(t, err)
+	assert.Equal(t, f[19:], []byte("Z E! TEST\n"))
+}
+
+func BenchmarkTelegrafLogWrite(b *testing.B) {
+	var msg = []byte("test")
+	var buf bytes.Buffer
+	w := newTelegrafWriter(&buf)
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		w.Write(msg)
+	}
+}