title: how enable requests async mode?
body: <p>for this code:</p>
  
  <pre><code>import sys
  
  import gevent
  from gevent import monkey
  
  monkey.patch_all()
  
  import requests
  import urllib2
  
  def worker(url, use_urllib2=False):
      if use_urllib2:
          content = urllib2.urlopen(url).read().lower()
      else:
          content = requests.get(url, prefetch=True).content.lower()
      title = content.split('&lt;title&gt;')[1].split('&lt;/title&gt;')[0].strip()
  
  urls = ['http://www.mail.ru']*5
  
  def by_requests():
      jobs = [gevent.spawn(worker, url) for url in urls]
      gevent.joinall(jobs)
  
  def by_urllib2():
      jobs = [gevent.spawn(worker, url, True) for url in urls]
      gevent.joinall(jobs)
  
  if __name__=='__main__':
      from timeit import Timer
      t = Timer(stmt="by_requests()", setup="from __main__ import by_requests")  
      print 'by requests: %s seconds'%t.timeit(number=3)
      t = Timer(stmt="by_urllib2()", setup="from __main__ import by_urllib2")  
      print 'by urllib2: %s seconds'%t.timeit(number=3)
      sys.exit(0)
  </code></pre>
  
  <p>this result: </p>
  
  <pre><code>by requests: 18.3397213892 seconds
  by urllib2: 2.48605842363 seconds
  </code></pre>
  
  <p>in sniffer it looks this:</p>
  
  <p><img src="https://i.stack.imgur.com/IsOgD.jpg" alt="">
  description: first 5 requests are sended by requests library, next 5 requests are sended by urllib2 library.
  red - is time when work was freeze, dark - when data receiving... wtf?!</p>
  
  <p><strong>How it posible if socket library patched and libraries must work identically?
  How use requests without requests.async for asynchronious work?</strong></p>