title: how to give a name to downloaded file with async tornado client
body: <p>i have some python tornado code that i use to get files async</p>
  
  <pre><code>from tornado import ioloop , httpclient
  def get_file(url):
      http_client = httpclient.AsyncHTTPClient()
      http_client.fetch(url,done)
  
  def done()
      print "Done"
  url = "http://localhost/files/ldfhgdksgfjkdfadf/output.pdf"
  ioloop.IOLoop.instance().start()
  </code></pre>
  
  <p>what happens is that the file gets saved in the current dir as "output.pdf", is there any way with asynchttp client to specify a file name? if i could i would call the file by some name and save it in a different directory other than the current directory.</p>