Feature/hadoop

For context, see https://groups.google.com/forum/#!topic/zipkin-user/Z89XJfC3goo
The commit messages explain a bit what's going on in the PR. It's basically just a copy of [this Scalding job](https://gist.github.com/bmdhacks/48f1108a5577dbd2fa3a) discussed in https://groups.google.com/forum/#!msg/zipkin-user/q_EMWaNbHCw/qBCHqgeL3bMJ, with a custom Scalding tap/sink built on top of zipkin-cassandra.

I chose to put the low-level hadoop implementation in zipkin-cassandra, since it's cassandra-specific. That also makes stuff easier if Zipkin is ever ported to the native Cassandra client, since Cassie is deprecated. (see https://groups.google.com/forum/#!topic/zipkin-user/Ul58F0WZfe4)

For some reason, I had to change some stuff in zipkin-scrooge to make it build with sbt. I'm not quite sure why. I included it in this PR as a separate commit. If one can build 3b1e4bf without a493c0b, then I guess one could drop that one.

The module is currently only built with sbt - it's missing pants BUILD files. Somebody with more pants experience has to add that, I guess.

There will also be a web UI, but I'll provide that as a second PR later on, so the PR doesn't grow too big!

There's a README showing how to set it up. It would be great if people who run Zipkin with Cassandra in production could battle-test this, and reply to the PR!
actually, if you are on a recent version of scalding, .group.sum == .sumByKey