Add feature for safely accessing related attributes

[robinzachmann]

The problem

foo = Foo.objects.all().get()
foo.bar  # <-- this will silently cause a sub-query

foo = Foo.objects.all().get()
foo.bar  # <-- this will silently cause a sub-query

Idea

Create a function for safely accessing foo.bar which complains if the attribute would cause an extra query.