Add forkIO for Future

Signature:

    def forkIO[A](a: IO[A])(implicit ec: ExecutionContext)

Usage:

    import scalaz.contrib.std.scalaFuture.forkIO
    import scalaz.effect.IO

    // Non-determinism
    val program = for {
      _ <- forkIO(IO.putStrLn("Hello"))
      _ <- forkIO(IO.putStrLn("World"))
      _ <- forkIO(IO.putStrLn("!"))
    } yield ()

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This adds a dependency on scalaz-effect.