fs: make mkdir() default to 0777 permissions

Fixes #1999.

lib/fs.js

@@ -354,11 +354,11 @@ fs.fsyncSync = function(fd) {
 };
 
 fs.mkdir = function(path, mode, callback) {
-  binding.mkdir(path, modeNum(mode), callback || noop);
+  binding.mkdir(path, modeNum(mode, 511 /*=0777*/), callback || noop);
 };
 
 fs.mkdirSync = function(path, mode) {
-  return binding.mkdir(path, modeNum(mode));
+  return binding.mkdir(path, modeNum(mode, 511 /*=0777*/));
 };
 
 fs.sendfile = function(outFd, inFd, inOffset, length, callback) {