add var self = this to progress bar

Fixes crash when using progress bar:
	ReferenceError: self is not defined
	at ProgressBar.<anonymous>
	(/home/cbryant/bl/node_modules/blessed/lib/widget.js:4862:11)
	...
						v

lib/widget.js

@@ -4832,6 +4832,8 @@ Button.prototype.press = function() {
  */
 
 function ProgressBar(options) {
+  var self = this;
+
   if (!(this instanceof Node)) {
     return new ProgressBar(options);
   }