Allow any instance of Client to be passed in.

Added new example for publishing in callback.

PubSubClient/examples/mqtt_auth/mqtt_auth.ino

@@ -1,7 +1,8 @@
 /*
- Basic MQTT example 
+ Basic MQTT example with Authentication
  
-  - connects to an MQTT server
+  - connects to an MQTT server, providing username
+    and password
   - publishes "hello world" to the topic "outTopic"
   - subscribes to the topic "inTopic"
 */
@@ -19,7 +20,8 @@ void callback(char* topic, byte* payload, unsigned int length) {
   // handle message arrived
 }
 
-PubSubClient client(server, 1883, callback);
+EthernetClient ethClient;
+PubSubClient client(server, 1883, callback, ethClient);
 
 void setup()
 {