refactor(json): make jsonite optional with build tags (#1026)

* refactor(json): Restore gin support for app engine

Create new folder to support multiple json package.
restore gin support for app engine (disable jsonite through tags)

use jsoniter

$ go build -tags=jsoniter .

use default json

$ go build .

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* rename json file.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* docs(json): add build tags document.

* fix(docs): markdown format.

* fix(json): missing space.

json/json.go

@@ -0,0 +1,17 @@
+// Copyright 2017 Bo-Yi Wu.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+// +build !jsoniter
+
+package json
+
+import (
+	"encoding/json"
+)
+
+var (
+	Marshal       = json.Marshal
+	MarshalIndent = json.MarshalIndent
+	NewDecoder    = json.NewDecoder
+)

json/jsoniter.go

@@ -0,0 +1,18 @@
+// Copyright 2017 Bo-Yi Wu.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+// +build jsoniter
+
+package json
+
+import (
+	"github.com/json-iterator/go"
+)
+
+var (
+	json          = jsoniter.ConfigCompatibleWithStandardLibrary
+	Marshal       = json.Marshal
+	MarshalIndent = json.MarshalIndent
+	NewDecoder    = json.NewDecoder
+)