added an async implementation that boxes on return

Because nancy expects an object to do with as she may see fit and
because we want to handle our routes in an async context we added the
box function to the return function of the AsyncBuilder. A fancy
function now has the signature NancyModule -> 'a -> Async<obj>, but you
can choose to return a string, a Nancy Negotiator, JSON or whatever
// Nancy has a serializer for.