Draft of simple core.async recipe.

Known issues...

- Problem and Solution feel clunky to me. I kind of want to write the problem statement as: "You want to play with core.async", because, let's face it. 

- How are we handling dependency versions that will be woefully outdated by publication time?

- There's no high-level "what is core.async" business here. Not sure if that's covered elsewhere or if this is a decent place to do it.

- The example problem in this recipe lends itself to a Round 2 wherein we introduce pub-sub. Should that be a second recipe? I think that would make sense, though I'd hate for people to scan it and assume core.async is "just for doing pub-sub", and that would de-simplify this intro recipe a bit.