Add integration tests, fix a minor bug en route

The spec says empty bodies may not have a signature. Previously this
would crash the non-streaming parser (the streaming equivalent already
handled this). That's now fixed.

Tests are intended to provide a basic integration coverage over the
whole project so we can change things with some idea that the whole
setup still works.

Author: pimterry

lib/message.js

@@ -72,7 +72,10 @@ module.exports.unmarshall = function unmarshall(buff, opts) {
   message.flags = headers[2];
   message.serial = headers[5];
   msgBuf.align(3);
-  message.body = msgBuf.read(message.signature);
+  var bodyLength = headers[4];
+  if (bodyLength > 0 && message.signature) {
+    message.body = msgBuf.read(message.signature);
+  }
   return message;
 };