Add server property to the Message class

Extremely simple change, but I (and others) would probably find it a
nice shortcut.

lib/Structures/Message.js

@@ -40,6 +40,7 @@ var Message = (function (_Equality) {
 
 		_Equality.call(this);
 		this.channel = channel;
+		this.server = channel.server;
 		this.client = client;
 		this.nonce = data.nonce;
 		this.attachments = data.attachments;