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/README.md

@@ -0,0 +1,5 @@
+# Graylog Output Plugin
+
+This plugin writes to a Graylog instance using the "gelf" format.
+
+It requires a `servers` name.