Add support for async wrapped handlers - #23

I guess the difference between this and #22 is that this does not require a different mechanism for executing the wrapper (via runAsync), as it will continue to work as expected via the normal run command.

Could you pls add a test for a case when I use a callback at async handler, see:

const testMod8 = {
  handler: async (event, context, callback) => {
    if (event.test === "success") {
      callback(null, "Success");
    }
    if (event.test === "fail") {
     callback(new Error("Fail"));
    }
  }
};

PS: nobody force me to get rid of a callback in case of async handler. In other words, you never know does function use callback or not.

@dhm116 Thanks for the PR. Included in lambda-wrapper 0.3.0

@Enase that scenario you're describing would be mixing promises and callbacks, which is never recommended anyhow - I don't mind creating a follow-up PR with additional test coverage around that scenario if others think it would be beneficial...

@Enase I had the same concern as @dhm116 , i.e. it is difficult to see a real-life / best practice scenario where you would use async with callbacks instead of Promises hence I did not include the proposed change in the merge

@mpuittinen I totally agree with you. but in a real life code is written using async handlers for nodejs6.10. Most of people just changed babel config options and now we have async handlers that use callbacks. My goal is just to point you on a trouble. Just imagine how many people will copy paste the example from serverless blog or Serverless Node.js Starter and many and many other places.
Once again, in general you are absolutely right. I won't going to kill your time anymore.

@Enase true, the blog / starter may drive people to "bad" practices. Since it's just a question of test cases (no code changes on the actual modules), I've added those.