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 
I added unit tests for the utility functions, and the SpanStoreValidator has unit tests for the MongoDB accessing code.