Add HBase backend

* Added zipkin-hbase subproject
* Created HTableFactoryWithExecutor to create HTables with a thread pool.
* Create HBaseTable, an async facade on HTable.
* Created HBaseStorage
* HBase storage puts serialized thrift into the zipkin.storage table
* Created HBaseIndex
* add the ability to globally assign an id to a string in HBase.
* Add hbase command to query and collector
* Changed indexSpanDuration to return Future[Unit].