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 
WTF? Is this code copied from one of the other files? If so, can you document which?