Add gelf serializer & graylog output filter. (#1167)

* add gelf serializer.

* change url.

* handle fields in correct format.

* add graylog.

* handle host field of graylog.

* 1: Add go-gelf entry to Godeps to fix ci.
2: switch to github.com/Graylog2/go-gelf.

* implement Close().

* Deprecated gelf serializer, and back to graylog-golang.

* Update graylog-golang's hash.

* move gelf related function to graylog.go.

* 1: remove uneeded deps on Godeps_windows.
2: add README.md
3: add unittest.

* Fix unittest on 'go test -race'

plugins/outputs/graylog/graylog_test.go

@@ -0,0 +1,55 @@
+package graylog
+
+import (
+	"bytes"
+	"compress/zlib"
+	"encoding/json"
+	"io"
+	"net"
+	"sync"
+	"testing"
+
+	"github.com/influxdata/telegraf/testutil"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestWrite(t *testing.T) {
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go UDPServer(t, &wg)
+
+	i := Graylog{
+		Servers: []string{"127.0.0.1:12201"},
+	}
+	i.Connect()
+
+	metrics := testutil.MockMetrics()
+	metrics = append(metrics, testutil.TestMetric(int64(1234567890)))
+
+	i.Write(metrics)
+
+	wg.Wait()
+	i.Close()
+}
+
+type GelfObject map[string]interface{}
+
+func UDPServer(t *testing.T, wg *sync.WaitGroup) {
+	serverAddr, _ := net.ResolveUDPAddr("udp", "127.0.0.1:12201")
+	udpServer, _ := net.ListenUDP("udp", serverAddr)
+	defer wg.Done()
+
+	bufR := make([]byte, 1024)
+	n, _, _ := udpServer.ReadFromUDP(bufR)
+
+	b := bytes.NewReader(bufR[0:n])
+	r, _ := zlib.NewReader(b)
+
+	bufW := bytes.NewBuffer(nil)
+	io.Copy(bufW, r)
+	r.Close()
+
+	var obj GelfObject
+	json.Unmarshal(bufW.Bytes(), &obj)
+	assert.Equal(t, obj["_value"], float64(1))
+}