Feature/22 transaction support

[WojciechMazur]

Introduces transactional operations using Mutatus as it is supported in Datastore.
It does not break backward compability due to introduction of Contexts. Operations executed with default mode (not transactional or batched) are handled with implicit Context.Default

In case if all operations within transaction block would pass, transaction should be automatically commited, in other case whole transaction would be rolled back

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

Also changed return type of QueryBuilder.run() to Stream[Result[T]]
from previous Result[Stream[Result[T]]]

Using the same syntax is possible to execute Batch, using Dao.batch(implicit batch => ..., which is write only operation. In case of using read operations within batch block compile time error would be thrown.

[WojciechMazur]

This one is a bit dirty hack, but I've not found better solution to check at compile time if prohibited read operations were used.