Support for async/await main handler

[jeremydaly]

Now that Lambda supports v8.10, we should be able to declare our main handler like this:

exports.handler = async (event, context) => {
  // Run the request
  return api.run(event, context)
}

This should be optional for backwards compatibility.

[jeremydaly]

Added in some magic to make this happen, but if used, route functions need to use async/await. Might need some more testing.