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
```

Makefile

@@ -22,6 +22,7 @@ assets:
 
 test:
 	@(go test -race -v github.com/minio/mcs/restapi/...)
+	@(go test -race -v github.com/minio/mcs/pkg/auth)
 
 coverage:
 	@(go test -v -coverprofile=coverage.out github.com/minio/mcs/restapi/... && go tool cover -html=coverage.out && open coverage.html)