Add QuerySetFilter

I was wondering if you're interested in something like this QuerySetFilter. It allows easy access to methods of custom QuerySets. It might be interesting due to the QuerySet/Manager updates in 1.7, and I think it can lead to less and better organized code. If yes I'd look into adding tests and a little documentation.

Usage is basically
```
    qs_choices = {
        '': ('---------', {
            'method': '',
        }),   
        'foo': ('Foo', {
            'method': 'foo',
        }),   
        'bar': ('Bar', {
            'method': 'bar',
            'args': (True, 5, 'baz'),
        }),   
    }
    qs_filter = QuerySetFilter(qs_choices, label='QuerySet choices')
```