Enhancement: inject logger object into backend

* allows backends to use/consume the configured logger from statsd
 * this is optional, backends who don't use it yet, should continue to work
 * related: librato/statsd-librato-backend#17

stats.js

@@ -39,7 +39,7 @@ function loadBackend(config, name) {
     l.log("Loading backend: " + name, 'DEBUG');
   }
 
-  var ret = backendmod.init(startup_time, config, backendEvents);
+  var ret = backendmod.init(startup_time, config, backendEvents, l);
   if (!ret) {
     l.log("Failed to load backend: " + name);
     process.exit(1);