PushMessageData cleanup

- "Objects implementing PushMessageData gain an associated bytes (a byte
sequence, initially empty)." -> "PushMessageData objects have an associated
bytes", as there are no other objects implementing this.

- Made PushEventInit non-optional and its data member required.

As suggested on https://github.com/w3c/push-api/pull/88#issuecomment-63685522

- Also fixed linkification of PushEvent.data.