STS integration, JWT auth and Stateless MCS (#70)

This commit changes the authentication mechanism between mcs and minio to an sts
(security token service) schema using the user provided credentials, previously
mcs was using master credentials. With that said in order for you to
login to MCS as an admin your user must exists first on minio and have enough
privileges to do administrative operations.

```
./mc admin user add myminio alevsk alevsk12345
```

```
cat admin.json

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "admin:*",
        "s3:*"
      ],
      "Resource": [
        "arn:aws:s3:::*"
      ]
    }
  ]
}

./mc admin policy add myminio admin admin.json
```

```
./mc admin policy set myminio admin user=alevsk
```

restapi/user_logout.go

@@ -17,14 +17,13 @@
 package restapi
 
 import (
-	"errors"
+	"log"
 
 	"github.com/go-openapi/runtime/middleware"
 	"github.com/go-openapi/swag"
 	"github.com/minio/mcs/models"
 	"github.com/minio/mcs/restapi/operations"
 	"github.com/minio/mcs/restapi/operations/user_api"
-	"github.com/minio/mcs/restapi/sessions"
 )
 
 func registerLogoutHandlers(api *operations.McsAPI) {
@@ -38,21 +37,23 @@ func registerLogoutHandlers(api *operations.McsAPI) {
 	})
 }
 
-// logout() deletes provided bearer token from in memory sessions map
-// then checks that the session actually got removed
-func logout(sessionID string) error {
-	sessionsMap := sessions.GetInstance()
-	sessionsMap.DeleteSession(sessionID)
-	if sessionsMap.ValidSession(sessionID) {
-		return errors.New("something went wrong deleting your session, please try again")
-	}
-	return nil
+// logout() call Expire() on the provided minioCredentials
+func logout(credentials MCSCredentials) {
+	credentials.Expire()
 }
 
 // getLogoutResponse performs logout() and returns nil or error
-func getLogoutResponse(sessionID string) error {
-	if err := logout(sessionID); err != nil {
+func getLogoutResponse(jwt string) error {
+	creds, err := getMcsCredentialsFromJWT(jwt)
+	if err != nil {
+		log.Println(err)
 		return err
 	}
+	credentials := mcsCredentials{minioCredentials: creds}
+	if err != nil {
+		log.Println("error creating MinIO Client:", err)
+		return err
+	}
+	logout(credentials)
 	return nil
 }