Include an application ID

[haydn]

The Payload should have an appId added to it:

type Payload = {|
  // The app ID identifies a unique app instance or isolate app environments.
  appId: string,
  // The write key will be exposed to the public, but it can be easily revoked
  // if someone start abusing it.
  writeKey: string,
  // In the future, we can do some smart batching, but for now this will always
  // contain exactly 1 event.
  events: [Event],
|};