Add hadoop job for analyzing spans

- Implement SpanSource for Cassandra
- Fix bug in zipkin-cassandra where Dependencies
  weren't stored correctly
- Add Stopwatch.java - a hacky solution to some
  classpath problem/MethodNotFoundException
- New module zipkin-aggregate with Scalding job
- Readme file for running the job

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=546952