Add forkIO for Future

[puffnfresh]

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.

[larsrh]

Great, thanks!