Remove grunt.util._ / Add "default" task 

* [`grunt.util._`](http://gruntjs.com/api/grunt.util#grunt.util._) is [deprecated](http://gruntjs.com/blog/2013-11-21-grunt-0.4.2-released). So I replaced it with [lodash](https://github.com/lodash/lodash) module.
* I added the `default` task, which runs a test, to the `Gruntfile.coffee` file. Now we can run the test for this plugin with `npm test` command.