Add Create Service Account api (#72)

adds new functionality for creating a service
account for a user, for this, an admin client
is created with the user credentials so that
the service account can be assigned to him.

This also updates to  minio RELEASE.2020-04-28T23-56-56Z

pkg/auth/jwt.go

@@ -52,6 +52,7 @@ func IsJWTValid(token string) bool {
 	return err == nil
 }
 
+// DecryptedClaims claims struct for decrypted credentials
 type DecryptedClaims struct {
 	AccessKeyID     string
 	SecretAccessKey string