Add optional queryset approach on data-dependent methods

Fix #219. Instead of having each method take a processor as was outlined in the original ticket, I have them take an optional queryset argument. This is much more generic than a processor or processor name but can still be used in the case of a custom processor by supplying these methods with the returned value of the `get_queryset(...)` method on the processor.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
This (and all others below) should be `if queryset is None` otherwise the queryset will be [evaluated](https://docs.djangoproject.com/en/1.6/ref/models/querysets/#when-querysets-are-evaluated) as a boolean.