Add support async scripts

/to @bajtos 
/cc @ritch 

The PR adds two enhancements for boot:

Allow boot scripts to be executed asynchronously
> module.exports = function(app, callback) {...}

This should not be needed any more, please simplify to

```js
process.nextTick(function() {
  process.bootFlags.barProcessed = true;
  callback();
});
```