title: grunt-contrib-watch doesn't work in every directory level
body: <p>I'm trying to watch my app's <em>js</em> files and dependencies's <em>css &amp; js</em> files for link them in my index.html file.</p>
  
  <p>I'm using <strong>grunt-contrib-watch</strong> for the job, but it seems that it doesn't work as I expected. There are directories in the file system tree that doesn't trigger the watch task.</p>
  
  <p>this is my grunt watch config:</p>
  
  <pre><code>watch: {
              vendors: {
                  files: ['www/vendors/**/*.js', 'www/vendors/**/*.css'],
                  tasks: ['wiredep']
              },
              app: {
                  files: ['www/app/**/*.js', 'www/app/**/*.css', 'www/css/**/*.css'],
                  tasks: ['tags']
              }
          }
  </code></pre>
  
  <p>this is my folders structure:</p>
  
  <pre><code>    C:.
      └───www
          ├───app
          │   └───modules
          │       └───main
          │           └───controllers
          ├───css
          ├───images    
          └───vendors
              ├───angular
              ├───bootstrap
              │   ├───dist
              │   │   ├───css
              │   │   ├───fonts
              │   │   └───js
              │   ├───fonts
              │   ├───grunt
              │   ├───js
              │   └───less
              └───jquery
                  ├───dist
                  └───src
                      ├─── ...                
  </code></pre>
  
  <p>Here is the PROBLEM. When I add/delete a js or css file to a directory that <strong>already have js/css file</strong>, the watcher task triggers and it works. But any other folder that doesn't have js/css placed in (and the folder is placed in <strong>vendors</strong> or <strong>app</strong> folders), doesn't trigger the watcher, and the task won't run.</p>
  
  <p>Why is that? and how can I solve it? I want the watcher to work on any level in the directory's tree.</p>
  
  <p>I'm using grunt 0.4.5, and grunt-contrib-watch version 0.6.1.</p>