Add descending field options to order_by when using order_by=True

Fixes #91
Fix (fltr.name or f) in string format.
User friendly descending labels

and switched from list comprehension to regular loop for readability.
Translation marker for '%s descending'.
Don't need lazy translation.
User friendly descending label for order_by

and added test.
Merge branch 'develop' into nicholasserra-bug/descending-order-by