title: boost::filesystem get relative path
body: <p>What methods of the <code>boost::filesystem</code> library can  help me to get a path relative to another path?</p>
  
  <p>I have a path <code>/home/user1/Downloads/Books</code> and a path <code>/home/user1/</code>. Now I want to get a path <code>Downloads/Books</code>.</p>