Expose API in addition to the command-line option

[colinmathews]

Nice little utility here.

This PR wraps deploy.js in a module and exposes it so that it can be called programmatically. deploy.js can still be run as a command-line utility.

The API uses a standard callback with arguments (error, result), but for now the error property is always null. Further work could switch and/or catch thrown exceptions and feed them into the error argument (instead of just throwing them).

The result to the callback is an object instead of a status so it can easily be extended in the future:

{
  status: '...'
}

Here's a quick sample of how the call works:

var deployer = require('opsworks-deploy');
deployer.run({
  accessKeyId: '...',
  secretAccessKey: '...',
  region: '...',
  stackId: '...',
  layerId: '...',
  appId: '...',
  rolling: true|false
}, (err, result) => {
  if (err) {
    ...
  }
  else {
    ...
  }
});

Cheers!