title: Add paperclip new style doesn't affect old uploaded images
body: <p>I have Company model with logo image</p>
  
  <pre><code>has_attached_file :logo
  </code></pre>
  
  <p>I have created many companies with their logos.</p>
  
  <p>Now, I need to add new style</p>
  
  <pre><code>has_attached_file :logo, :styles =&gt; { :small =&gt; "30x15&gt;", :medium =&gt; "155x85&gt;" }
  </code></pre>
  
  <p>Should I re-upload all my old data to regenerate the new styles? I don't think so....
  Or is there any rake task can regenerate styles?</p>