Add injection of Map[String, List[Long]] for span duration aggregation

In order to support span duration in the summingbird job, an injection of
Map[String, List[Long]] is added. Summingbird job can aggregate the duration of
the spans(List[Long]) and come up with stats such as avg, p99 etc

Author: @suchang
Fixes #353
URL: https://github.com/twitter/zipkin/pull/353