+- the sum filter can now sum up values by attribute. This is a backwards
+ incompatible change. The argument to the filter previously was the
+ optional starting index which defaultes to zero. This now became the
+ second argument to the function because it's rarely used.
+- like sum, sort now also makes it possible to order items by attribute.
+- like sum and sort, join now also is able to join attributes of objects
+ as string.
+- the internal eval context now has a reference to the environment.
+- added a mapping test to see if an object is a dict or an object with
+ a similar interface.