title: Grunt Copy Flatten not working as expected
body: <p>I have a directory structure as follows:</p>
  
  <pre><code>source/
      libraries/
          d3.js
          lodash.js
          //etc
  </code></pre>
  
  <p>I have grunt-copy setup as follows:</p>
  
  <pre><code>copy: {
    main: {
      files: [
        {
          src: ["source/libraries/*.js"], 
          dest: "build/", 
          flatten: true
        }
  </code></pre>
  
  <p>I expect it to flatten the output into build, so that I will have </p>
  
  <pre><code>build/
      d3.js
      //etc
  </code></pre>
  
  <p>Instead, I get a reproduction of the original directory structure in build:</p>
  
  <pre><code>build/
      source/
          libraries/
              d3.js
              //etc
  </code></pre>
  
  <p>What gives? Am I not using flatten properly?</p>