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
Add web UI to aggregates

Draws a directed graph that shows the dependencies between
services. Based upon aggregated data from the Scalding
job in zipkin-aggregate.

- Add new web endpoint, /aggregate
- Import dagre-d3 via bower
- Add javacript to render JSON result from /api/dependencies