title: use PHPExcel with composer and Symfony2.2
body: <p>I found this on SO: <a href="https://stackoverflow.com/questions/6402839/how-to-use-phpexcel-correctly-with-symfony-2">How to use PHPExcel correctly with Symfony 2</a></p>
  
  <p>This works, but I want to use it with composer.
  The first part I already solved: to load PHPExcel for a special tag (the last stable release)</p>
  
  <p>I don't find out how to fetch a tag with this syntax:</p>
  
  <pre><code>"repositories": [
      {
          "type": "vcs",
          "url": "https://github.com/umpirsky/SyliusAssortmentBundle"
      }
  ]
  </code></pre>
  
  <p>So I use the <code>Package</code> notation:<br>
  I found out, the <code>reference</code> should be the tag name on github.
  And the <code>version</code> cannot be the same value (PHPExcel_1.7.8). Seems that alphabetical characters are not allowed, so it's only the version as a number (1.7.8)</p>
  
  <pre><code>"repositories": [{
      "type": "package",
      "package": {
          "name": "PHPOffice/PHPExcel",
          "version": "1.7.8",
          "source": {
              "url": "https://github.com/PHPOffice/PHPExcel.git",
              "type": "git",
              "reference": "PHPExcel_1.7.8"
          }
      }
  }]
  </code></pre>
  
  <p>The next step I didn't solve. I tried out every combination for the autoloading: psr-0, classmap, different paths, relative to project/vendor/phpexcel, update composer everytime, but nothing worked.</p>
  
  <p>It only works, if I put this line</p>
  
  <pre><code>$loader-&gt;add('PHPExcel', __DIR__.'/../vendor/PHPOffice/PHPExcel/Classes');
  </code></pre>
  
  <p>into the app/autoload.php. I found out, that the first string (<code>PHPExcel</code>) can also be an empty string: <code>''</code>.<br>
  Is there a differnece if I use <code>PHPExcel</code> or <code>''</code>?</p>
  
  <p><strong>So my primary question is</strong>, how can I avoid to write this line into the autoload.php, put the equivalent commands into my project's composer.json?</p>