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

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