Remove grunt.util.async and grunt.util._

[`grunt.util.async`](http://gruntjs.com/api/grunt.util#grunt.util.async) and [`grunt.util._`](http://gruntjs.com/api/grunt.util#grunt.util._) are [deprecated](http://gruntjs.com/blog/2013-11-21-grunt-0.4.2-released).
So I replaced them with [async](https://github.com/caolan/async) module and [extend](https://github.com/justmoon/node-extend) module.