Add caching to the changes api since it is so expensive and can return large results.

util/cache.py

@@ -0,0 +1,12 @@
+from functools import wraps
+
+
+def cache_control(max_age=55):
+  def wrap(f):
+    @wraps(f)
+    def add_max_age(*args, **kwargs):
+      response = f(*args, **kwargs)
+      response.headers['Cache-Control'] = 'max-age=%d' % max_age
+      return response
+    return add_max_age
+  return wrap