Pass backoff options from constructor

The Client constructor supports backoff configuration. This commit
exposes it to the end developer via the `peers` options object, in the
`backoff` property.
Example:

Gun({
  peers: {
    'url.com/gun': {
      backoff: { max: 1000, factor: 0.2 }
    }
  }
})

gun.js

@@ -2412,7 +2412,10 @@
 					return;
 				}
 
-				var client = Client.pool[url] = new Client(url);
+				var client = new Client(url, options.backoff);
+
+				// Add it to the pool.
+				Client.pool[url] = client;
 
 				// Listen to incoming messages.
 				client.on('message', function (msg) {