title: emacs + dirtree work like normal file browser
body: <p>I am new to emacs, and I tried dirtree like this picture:
  I use <code>m-x dirtree</code> and it shows up the left frame.
  <a href="http://db.tt/IbN9UaJR">http://db.tt/IbN9UaJR</a></p>
  
  <p>But when I click on the file, it open the file in left frame/buffer?...
  the file content replace the dirtree itself.</p>
  
  <p>and I want it works like normal file browser like textwranger:
  <a href="http://db.tt/H14KsUsD">http://db.tt/H14KsUsD</a></p>
  
  <p>How can I do this?
  Moreoever, How to make dirtree open automatically when start emacs?</p>