Implementation of statistics aggregators

This corresponds to do API calls on:
/v2/meters/meter-name/statistics?aggregate.func=func-name

Usage:
aggregates = [{'func': 'cardinality', 'param': 'resource_id'}])
client.statistics.list(meter_name="instance", aggregates=aggregates)

CLI:
ceilometer statistics -m instance -a "cardinality<-resource_id"

Change-Id: I0096668585a5c7e7985973f07049eb91f44413fe