Add Task packages as alternative to Async and Hopac

[panesofglass]

As ASP.NET Core and OWIN work directly with Task, it seems worthwhile to try to leverage the TaskBuilder.fs, at least for the sake of some benchmarks to determine whether this provides a possible simplification to the codebase. A first pass is likely to make things harder, but I would hope to move in the direction of Giraffe with respect to supporting Task-only were we to find that it performed better than async and equivalent to Hopac. This would also help with #200, should we choose to go that route.