Add new aggregation API

Zeitgeist API can give us vague information but not statistics over the API although the info is there.
Use case:
- Give me counts of every subject_text from actor = Unity
Currently to do that one will need to either:
Request all events with Unity as an Actor and count the subject_text
This can be done much better IMHO
I am thinking of an Aggregation Extension.
But before we hack on that we need to agree if its necessary and how the API will look like.