No support for multiple subscriptions

There is a need to permit multiple subscriptions on a given topic:
```
        "Subscription": [
          {
           "Endpoint" : "user1@tld.com",
           "Protocol" : "email"
          },
          {
           "Endpoint" : "user2@xyz.com",
           "Protocol" : "email"
          }
        ]
```

Any thoughts on supporting this?