Add optional syslog logrus hook (#343)

* add optional syslog logrus hook

* document syslog

vendor/github.com/sirupsen/logrus/hooks/syslog/README.md

@@ -0,0 +1,39 @@
+# Syslog Hooks for Logrus <img src="http://i.imgur.com/hTeVwmJ.png" width="40" height="40" alt=":walrus:" class="emoji" title=":walrus:"/>
+
+## Usage
+
+```go
+import (
+  "log/syslog"
+  "github.com/sirupsen/logrus"
+  lSyslog "github.com/sirupsen/logrus/hooks/syslog"
+)
+
+func main() {
+  log       := logrus.New()
+  hook, err := lSyslog.NewSyslogHook("udp", "localhost:514", syslog.LOG_INFO, "")
+
+  if err == nil {
+    log.Hooks.Add(hook)
+  }
+}
+```
+
+If you want to connect to local syslog (Ex. "/dev/log" or "/var/run/syslog" or "/var/run/log"). Just assign empty string to the first two parameters of `NewSyslogHook`. It should look like the following.
+
+```go
+import (
+  "log/syslog"
+  "github.com/sirupsen/logrus"
+  lSyslog "github.com/sirupsen/logrus/hooks/syslog"
+)
+
+func main() {
+  log       := logrus.New()
+  hook, err := lSyslog.NewSyslogHook("", "", syslog.LOG_INFO, "")
+
+  if err == nil {
+    log.Hooks.Add(hook)
+  }
+}
+```

vendor/github.com/sirupsen/logrus/hooks/syslog/syslog.go

@@ -0,0 +1,55 @@
+// +build !windows,!nacl,!plan9
+
+package syslog
+
+import (
+	"fmt"
+	"log/syslog"
+	"os"
+
+	"github.com/sirupsen/logrus"
+)
+
+// SyslogHook to send logs via syslog.
+type SyslogHook struct {
+	Writer        *syslog.Writer
+	SyslogNetwork string
+	SyslogRaddr   string
+}
+
+// Creates a hook to be added to an instance of logger. This is called with
+// `hook, err := NewSyslogHook("udp", "localhost:514", syslog.LOG_DEBUG, "")`
+// `if err == nil { log.Hooks.Add(hook) }`
+func NewSyslogHook(network, raddr string, priority syslog.Priority, tag string) (*SyslogHook, error) {
+	w, err := syslog.Dial(network, raddr, priority, tag)
+	return &SyslogHook{w, network, raddr}, err
+}
+
+func (hook *SyslogHook) Fire(entry *logrus.Entry) error {
+	line, err := entry.String()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Unable to read entry, %v", err)
+		return err
+	}
+
+	switch entry.Level {
+	case logrus.PanicLevel:
+		return hook.Writer.Crit(line)
+	case logrus.FatalLevel:
+		return hook.Writer.Crit(line)
+	case logrus.ErrorLevel:
+		return hook.Writer.Err(line)
+	case logrus.WarnLevel:
+		return hook.Writer.Warning(line)
+	case logrus.InfoLevel:
+		return hook.Writer.Info(line)
+	case logrus.DebugLevel, logrus.TraceLevel:
+		return hook.Writer.Debug(line)
+	default:
+		return nil
+	}
+}
+
+func (hook *SyslogHook) Levels() []logrus.Level {
+	return logrus.AllLevels
+}