add examples/threadpool.py

examples/threadpool.py

+import time
+import gevent
+from gevent.threadpool import ThreadPool
+
+
+pool = ThreadPool(3)
+start = time.time()
+for _ in xrange(4):
+    pool.spawn(time.sleep, 1)
+gevent.run()
+delay = time.time() - start
+print 'Running "time.sleep(1)" 4 times with 3 threads. Should take about 2 seconds: %.3fs' % delay