update readme with zapier shout out.
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
fix the conflict with the mock library
Merge pull request #26 from brad/fix_mock_conflict

fix the conflict with the mock library
Merge pull request #25 from suborbital/support-count

Support Count() aggregates for M2Ms.
Merge https://github.com/zapier/django-drip