BUG: pickling MemoizeFunc does not store timestamp

Rather than using __getstate__ to override pickle, we use reduce, to
make sure that the __init__ is called at unpickle time, in order to
create the directory needed.

joblib/memory.py

@@ -76,7 +76,7 @@ class JobLibCollisionWarning(UserWarning):
 
 
 ################################################################################
-# class `Memory`
+# class `MemorizedFunc`
 ################################################################################
 class MemorizedFunc(Logger):
     """ Callable object decorating a function for caching its return value 
@@ -182,6 +182,15 @@ def __call__(self, *args, **kwargs):
                 shutil.rmtree(output_dir, ignore_errors=True)
                 return self.call(*args, **kwargs)
 
+
+    def __reduce__(self):
+        """ We don't store the timestamp when pickling, to avoid the hash
+            depending from it.
+            In addition, when unpickling, we run the __init__
+        """
+        return (self.__class__, (self.func, self.cachedir, self.ignore, 
+                self.save_npy, self.mmap_mode, self._verbose))
+
     #-------------------------------------------------------------------------
     # Private interface
     #-------------------------------------------------------------------------