Transactional API support

[WojciechMazur]

It should be possible to perform transactional operations using Mutatus which Datastore support.
The most comprehensive solution should allow to use existing syntax (also to not break backward compability)
In case if all operations within transaction block would pass, transaction should be automatically commited, in other case whole transaction would be rolled back
Solution may have syntax as follows

 Dao.transaction{ implicit tx =>  for{
       foo <- Dao.all.filter(_.bar == "bar").run() //since run returns Result[T]
       bar  <- foo.map(_.copy(bar = "foo")).saveAll
      } yield bar
    }

Also additional Batch API (Write only) may be designed as it may use same pattern.