Add support for choosing instance based on layer.

Sometimes you just want to check the instance running cron. What happens then? Huh? Adam happens!
If we take a layer_id option, we can't memoize the instances array, since it is different between different values for layer_id.

I would prefer if this was moved to the `Layer` class. Perhaps as something like:

    stack = ...
    stack.layers[0].instances