PageAgent: fix for main script without extension

Fixed a bug where `node --debug app` caused node-inspector to add
a non-existent file `app` into the resource tree.

lib/PageAgent.js

@@ -51,12 +51,22 @@ PageAgent.prototype = {
         function(evaluateResult, cb) {
           cb(null, evaluateResult[0], evaluateResult[1]);
         },
+        this._resolveMainAppScript.bind(this),
         this._getResourceTreeForAppScript.bind(this)
       ],
       done
     );
   },
 
+  _resolveMainAppScript: function(startDirectory, mainAppScript, done) {
+    fs.stat(mainAppScript, function(err, stat) {
+      if (err && !/\.js$/.test(mainAppScript)) {
+        mainAppScript += '.js';
+      }
+      return done(null, startDirectory, mainAppScript);
+    });
+  },
+
   _getResourceTreeForAppScript: function(startDirectory, mainAppScript, done) {
     async.waterfall(
       [