Drop useless __nonzero__

Python uses (x.__len__() != 0) to implement bool(x) when x doesn't
define a __nonzero__ (__bool__ on Pyton 3).

Add tests to make sure this works.

src/z3c/batching/README.txt

@@ -14,6 +14,8 @@ Batch on empty root:
   >>> batch = Batch([], size=3)
   >>> len(batch)
   0
+  >>> bool(batch)
+  False
   >>> batch.firstElement
   Traceback (most recent call last):
   ...
@@ -75,6 +77,11 @@ for:
   >>> len(Batch(sequence, start=12, size=3))
   1
 
+Like any sequence, a non-empty batch is true-ish in a boolean context:
+
+  >>> bool(batch)
+  True
+
 You can also get an element by index, which is relative to the batch:
 
   >>> batch[0]

src/z3c/batching/batch.py

@@ -144,11 +144,6 @@ def __eq__(self, other):
     def __ne__(self, other):
         return not self.__eq__(other)
 
-    def __nonzero__(self):
-        return self._trueSize != 0
-
-    # XXX __bool__ on py3!
-
     def __repr__(self):
         return '<%s start=%i, size=%i>' % (
             self.__class__.__name__, self.start, self.size)