Feature/len (#68)

* Add __len__ to discrete

* Add __len__ to tuple

gym/spaces/discrete.py

@@ -21,3 +21,5 @@ class Discrete(Space):
         return "Discrete(%d)" % self.n
     def __eq__(self, other):
         return self.n == other.n
+    def __len__(self):
+        return  n