Fix run handler callback

[abetomo]

• Fix not explicitly exit when result is present
  • Because Node.js 4.3 sometimes truncate on the way if the result size is large
  • node-lambda run output truncated when output data is large #264
• Modify exitcode to positive integer
  • Because the value changes on Windows and Linux.
• Added bin/node-lambda test

[DeviaVir]

👍 that's a nice fix to have. I wonder about the http://docs.aws.amazon.com/lambda/latest/dg/nodejs-prog-model-context.html#nodejs-prog-model-context-properties callbackWaitsForEmptyEventLoop, should we support it?

[abetomo]

I have not used Lambda with callbackWaitsForEmptyEventLoop set to false and I will check it.
If it seems to be able to support, I will try to fix it. :-)