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 is bad. You should either clear `require.cache` or modify `simpleAppInstructions` to copy the files to a unique subdirectory in the sandbox. The latter is probably a better solution.