add examples/bench_sleep0.py

examples/bench_sleep0.py

+from time import time
+from gevent import sleep
+
+N = 10000
+
+start = time()
+for _ in xrange(N):
+    sleep(0)
+delta = time() - start
+print 'sleep(0): %.1f microseconds' % (delta*1000000.0/N)
+