update individual item for instance case

[AlexRiina]

The docs have

    @takes_instance_or_queryset
    def tighten_lug_nuts(self, request, queryset):
        queryset.update(lugnuts=F('lugnuts') - 1)

but this updates all values since queryset seems to ignore the _result_cache (as proven by the test in my first commit). I'm not sure filter(pk=...) is the right way to implement this behavior, but @takes_instance_or_queryset seems dangerous as it is now.

[crccheck]

neat. looks like the new model meta api in Django 1.8 breaks things though. I was trying to avoid a db lookup, but looking at your solution, it seems worth having for simpler code. Performance in the Django admin is kind of a joke anyways.

[AlexRiina]

I copied the _meta lookup from the QuerySetIsh so while it doesn't work, it isn't a regression fortunately. I looked around a little and didn't see a great way of getting the model from the instance. Perhaps self.get_queryset(pk=queryset) is best.

[AlexRiina]

Is the requirements.txt file relevant? It isn't getting executed by the setup, contains a hard coded version of django and other dependencies, and isn't used by tox. I think it can be included in the setup.py and tox if you'd like to keep it. It would probably have to have pretty wide ranges like Django>=1.5

[crccheck]

requirements.txt is for developers only, and has extra stuff. There's duplication between that, tox.ini, and install_requires that I don't know how to get rid of in a way that doesn't create obfuscation.

[crccheck]

I always pin exact versions in Python projects because writing I'm too lazy to write and don't like the look of django-extensions>=1.5.8,<1.6 and Python doesn't have 1.5.x, ^1.5.8 or ~1.5.8.

[AlexRiina]

updated to pin to django-extensions==1.5.9