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) {...}

Can you move the initialisation of `process.bootFlags` to the test? The test is already clearing it in `afterEach` hook, thus it makes a lot of sense to set it up in `beforeEach`.