MongoDB Span Store

This adds MongoDB as a new SpanStore in Zipkin (so that Tracing data can be stored an retrieved from MongoDB).

Note that Zipkin daemons (this restriction is not imposed on the clients) require Scala 2.9.2. Additionally, because this is part of a Twitter codebase, Twitter futures are used, as opposed to Scala futures.

Reviewers: @patrick-bohan @slustbader @htmldoug 
This seems like a very limited suite of tests, I would expect to see a unit test for every method on the MongoDB accessing code.