fail to verify RPC notifications

[battlmonstr]

When a notification message is read (type = 2), MPVerifyMessage is called on that message, and fails, because the logic for checking the notification should be different.

The notification has only 3 elements:
https://github.com/gabriel/MPMessagePack/blob/master/RPC/MPRPCProtocol.m#L83

And it has no message ID:
https://github.com/gabriel/MPMessagePack/blob/master/RPC/MPRPCProtocol.m#L98

P.S. There's MPVerifyRequest and MPVerifyResponse, it would be nice to have MPVerifyNotification as well.