Single threaded executor for Tut docs

tut.md

import scala.concurrent.Future
import java.util.concurrent.Executor
import scala.concurrent.ExecutionContext
//Please don't do this in production code!  It is used to execute
//the documentation code synchronously to generate more readable output
implicit val synchronousExecutionContext = ExecutionContext.fromExecutor(new Executor {
  def execute(task: Runnable) = task.run()
})