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 
These are all fairly simple operations. It would be easier to debug if it were all in a single block, rather than running on potentially 4 different threads. These maps and conversions are all simple operations that would be fine to run in a single future, rather than creating so many potential context switches.

This looks like a real doozy to try and debug.