Stop walk from descending into next directory

Added ability to stop the walk from descending into a given
directory.
You use this by calling  in either the 'directory' or 'name' event
handler. This will prevent the directory from being traversed.

lib/node-type-emitter.js

@@ -47,7 +47,11 @@
   function emitSingleEvents(emitter, path, stats, next, self) {
     var num = 1 + emitter.listeners(stats.type).length + emitter.listeners("node").length;
 
-    function nextWhenReady() {
+    function nextWhenReady(flag) {
+      if (flag) {
+        stats.flag = flag;
+        flagSet = flag;
+      }
       num -= 1;
       if (0 === num) { next.call(self); }
     }

lib/walk.js

@@ -14,6 +14,7 @@
 
   function appendToDirs(stat) {
     /*jshint validthis:true*/
+    if(stat.flag && stat.flag === NO_DESCEND) { return; }
     this.push(stat.name);
   }