Add conversions between std Future and Task

I imagine that many people introducing Task into their code bases that use scala.concurrent.Future have implemented something similar.

There may be risky execution semantics issues that I haven't thought of, so please review carefully.

cc @pchiusano or @runarorama
One last naming comment (sorry -- I know these feel nagging).  If we already have names like "fromDisjunction," then for consistency it makes sense to me to have "fromStdFuture" rather than "stdFutureToTask" (and it's shorter).