Add support for using signal.setitimer or the itimer module for blocking detection

The itimer module can be found here: http://github.com/slideinc/itimer
for versions of Python prior to 2.6

eventlet/hubs/hub.py

@@ -4,10 +4,15 @@ import traceback
 import warnings
 import signal
 
-HAS_ITIMER = False
+alarm_func = signal.alarm
+
 try:
-    import itimer
+    if hasattr(signal, 'setitimer'):
-    HAS_ITIMER = True
+        HAS_ITIMER = True
+    else:
+        import itimer
+        HAS_ITIMER = True
+        alarm_func = itimer.alarm
 except ImportError:
     pass