Build fixture in order of creation

That is, we increase a global counter every time the 'Set' class
is subclassed.

The effect is that within a given fixture set, subsets are created
in the order of class creation.

Previously, the order was unpredictable.
Fixes django tests
Merge branch 'build-fixture-in-order-of-creation' of git://github.com/malthe/distillery into malthe-build-fixture-in-order-of-creation
Bumps 0.4.3