Support Count() aggregates for M2Ms.
Support Count() aggregates for M2Ms.

Use <m2m>__count for a field name and then treat define the rest of the
rule normally. In QuerySetRule.apply, the queryset is annotated behind
the scenes and then the filter is applied.

Conflicts:
	drip/models.py

alexphelps/django-drip,rwillmer/django-drip,syphar/django-drip,nym/django-drip,cloudControl/django-drip,alexphelps/django-drip,zapier/django-drip,syphar/django-drip,zapier/django-drip,kronok/django-drip,cloudControl/django-drip,rwillmer/django-drip,nym/django-drip,kronok/django-drip