config/main: Re-write config files - add to new config v3

- New config format.

```
{
	"version": "3",
	"address": ":9000",
    "backend": {
          "type": "fs",
          "disk": "/path"
    },
	"credential": {
		"accessKey": "WLGDGYAQYIGI833EV05A",
		"secretKey": "BYvgJM101sHngl2uzjXS/OBF/aMxAN06JrJ3qJlF"
	},
	"region": "us-east-1",
	"logger": {
		"file": {
			"enable": false,
			"fileName": "",
			"level": "error"
		},
		"syslog": {
			"enable": false,
			"address": "",
			"level": "debug"
		},
		"console": {
			"enable": true,
			"level": "fatal"
		}
	}
}
```

New command lines in lieu of supporting XL.

Minio initialize filesystem backend.
~~~
$ minio init fs <path>
~~~

Minio initialize XL backend.
~~~
$ minio init xl <url1>...<url16>
~~~

For 'fs' backend it starts the server.
~~~
$ minio server
~~~

For 'xl' backend it waits for servers to join.
~~~
$ minio server
... [PROGRESS BAR] of servers connecting
~~~

Now on other servers execute 'join' and they connect.
~~~
....
minio join <url1> -- from <url2> && minio server
minio join <url1> -- from <url3> && minio server
...
...
minio join <url1> -- from <url16> && minio server
~~~

config-migrate.go

@@ -0,0 +1,98 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package main
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+
+	"github.com/minio/mc/pkg/console"
+	"github.com/minio/minio/pkg/probe"
+)
+
+func migrateConfig() {
+	// Purge all configs with version '1'.
+	purgeV1()
+	// Migrate version '2' to '3'.
+	migrateV2ToV3()
+}
+
+// Version '1' is not supported anymore and deprecated, safe to delete.
+func purgeV1() {
+	cv1, err := loadConfigV1()
+	if err != nil {
+		if os.IsNotExist(err.ToGoError()) {
+			return
+		}
+	}
+	fatalIf(err.Trace(), "Unable to load config version ‘1’.", nil)
+
+	if cv1.Version == "1" {
+		console.Println("Unsupported config version ‘1’ found, removed successfully.")
+		/// Purge old fsUsers.json file
+		configPath, err := getConfigPath()
+		fatalIf(err.Trace(), "Unable to retrieve config path.", nil)
+
+		configFile := filepath.Join(configPath, "fsUsers.json")
+		os.RemoveAll(configFile)
+	}
+	fatalIf(probe.NewError(errors.New("")), "Unexpected version found ‘"+cv1.Version+"’, cannot migrate.", nil)
+}
+
+func migrateV2ToV3() {
+	cv2, err := loadConfigV2()
+	if err != nil {
+		if os.IsNotExist(err.ToGoError()) {
+			return
+		}
+	}
+	fatalIf(err.Trace(), "Unable to load config version ‘2’.", nil)
+	if cv2.Version != "2" {
+		return
+	}
+	serverConfig.SetAddr(":9000")
+	serverConfig.SetCredential(credential{
+		AccessKeyID:     cv2.Credentials.AccessKeyID,
+		SecretAccessKey: cv2.Credentials.SecretAccessKey,
+	})
+	serverConfig.SetRegion(cv2.Credentials.Region)
+	serverConfig.SetConsoleLogger(consoleLogger{
+		Enable: true,
+		Level:  "fatal",
+	})
+	flogger := fileLogger{}
+	flogger.Level = "error"
+	if cv2.FileLogger.Filename != "" {
+		flogger.Enable = true
+		flogger.Filename = cv2.FileLogger.Filename
+	}
+	serverConfig.SetFileLogger(flogger)
+
+	slogger := syslogLogger{}
+	slogger.Level = "debug"
+	if cv2.SyslogLogger.Addr != "" {
+		slogger.Enable = true
+		slogger.Addr = cv2.SyslogLogger.Addr
+	}
+	serverConfig.SetSyslogLogger(slogger)
+
+	err = serverConfig.Save()
+	fatalIf(err.Trace(), "Migrating from version ‘"+cv2.Version+"’ to ‘"+serverConfig.GetVersion()+"’ failed.", nil)
+
+	console.Println("Migration from version ‘" + cv2.Version + "’ to ‘" + serverConfig.GetVersion() + "’ completed successfully.")
+}