wire: create `shutdown` message type

In the wire project, create the `shutdown` message type. 

- This type should include properties as defined in [BOLT #2]( https://github.com/lightningnetwork/lightning-rfc/blob/master/02-peer-protocol.md#closing-initiation-shutdown).  
- It should implement `IWireMessage` and `deserialize` and `serialize` methods.  
- It should include basic unit tests for these methods
