Added aggregation 'count' for carbon-aggregation-rule.

It allows to get the number of times a metrics got pushed to the carbon aggregator.

It can be useful for instance in the case of an application pushing the execution time of every GET rest calls: apps.rest_service.GET.exec_time

    From this metric, the aggregator can build the following metrics:
    apps.rest_service.GET.call_count = count apps.rest_service.GET.exec_time
    apps.rest_service.GET.avg_exec_time = avg apps.rest_service.GET.exec_time
    apps.rest_service.GET.min_exec_time = min apps.rest_service.GET.exec_time
    apps.rest_service.GET.max_exec_time = max apps.rest_service.GET.exec_time