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
```

go.mod

@@ -3,6 +3,7 @@ module github.com/minio/mcs
 go 1.14
 
 require (
+	github.com/dgrijalva/jwt-go v3.2.0+incompatible
 	github.com/elazarl/go-bindata-assetfs v1.0.0
 	github.com/go-openapi/errors v0.19.4
 	github.com/go-openapi/loads v0.19.5
@@ -12,11 +13,14 @@ require (
 	github.com/go-openapi/swag v0.19.8
 	github.com/go-openapi/validate v0.19.7
 	github.com/jessevdk/go-flags v1.4.0
+	github.com/json-iterator/go v1.1.9
 	github.com/minio/cli v1.22.0
 	github.com/minio/mc v0.0.0-20200415193718-68b638f2f96c
 	github.com/minio/minio v0.0.0-20200415191640-bde0f444dbab
 	github.com/minio/minio-go/v6 v6.0.53
+	github.com/satori/go.uuid v1.2.0
 	github.com/stretchr/testify v1.5.1
 	github.com/unrolled/secure v1.0.7
+	golang.org/x/crypto v0.0.0-20200214034016-1d94cc7ab1c6
 	golang.org/x/net v0.0.0-20200324143707-d3edc9973b7e
 )