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 
The only thing that's similar is that we both use a thread pool. The HBase driver doesn't use a ForkJoinPool, and it actually wraps the HBase client in an object that uses a thread pool, as opposed to just providing an interface to enqueue functions in the pool.

https://github.com/twitter/zipkin/blob/master/zipkin-hbase/src/main/scala/com/twitter/zipkin/storage/hbase/utils/HBaseTable.scala