Properly collapse nested fieldsets

I noticed a few things were happening with the collapsing nested fieldsets:

1. The top-most collapse button was targeting all children with `collapse` class.
2. `static_prefix` wasn't being passed to the fieldset template.
3. `{% if static_prefix %}` was evaluating to False on the first loop when using the zero-indexed `forloop.counter0`.

This should fix #129, but I've only tested it with the test models/admin @Talkless provided.