title: web.py url handling: multiple subapplication redirection
body: <p>When using web.py framework. you can redirect an url to a subapplication.
  e.g (code.py):</p>
  
  <pre><code>import web
  import subapp1
  
  urls = (
      "/sub1", subapp1.app,
      "/(.*)", "index"
  )
  ....
  </code></pre>
  
  <p>this is really straight forward.</p>
  
  <p>However, when writing subapp1.py which has its own url handlers, if i want to re-route some url, say '/sub2', to another subapplication (subapp2), i am failing.</p>
  
  <p>Formerly in subapp1.py</p>
  
  <pre><code>import web
  import subapp2
  
  urls = (
      "/sub2", subapp2.app,
      "/(.*)", "some_local_class"
  )
  ....
  </code></pre>
  
  <p>GET request to "/sub1/sub2/", is handled by "some_local_class" in supapp1.py. But i need this url is re-routed to subapp2.py.</p>
  
  <p>Is there anything i am missing? Or may be this is not recommended url handling methodology in web.py?</p>