set timestamp field true by default and also make it configurable

lib/winston-loggly.js

@@ -58,7 +58,7 @@ var Loggly = exports.Loggly = function (options) {
     bufferOptions: options.bufferOptions || {size: 500, retriesInMilliSeconds: 30 * 1000}
   });
 
-  this.timestamp = true;
+  this.timestamp = options.timestamp === false ? false : true;
   this.stripColors = options.stripColors || false;
 };