do not allow creation of pools with size==0

gevent/pool.py

@@ -209,7 +209,7 @@ GreenletSet = Group  # the old name; will be deprecated in the future
 class Pool(Group):
 
     def __init__(self, size=None, greenlet_class=None):
-        if size is not None and size < 0:
+        if size is not None and size < 1:
             raise ValueError('Invalid size for pool (positive integer or None required): %r' % (size, ))
         Group.__init__(self)
         self.size = size