title: Rename directory names recursively
body: <p>I would like to rename all directories (recursively) by changing all occurrences of spaces to underscores. E.g.</p>
  
  <p>Before change:</p>
  
  <pre><code>product images/
    2010 products/
    2011 products/
    2012 products/
  misc images/
  nav images/
  </code></pre>
  
  <p>(etc.)</p>
  
  <p>After change:</p>
  
  <pre><code>product_images/
    2010_products/
    2011_products/
    2012_products/
  misc_images/
  nav_images/
  </code></pre>
  
  <p>Any help is appreciated.</p>