Add Sass Grunt task

* Resolves sensu/uchiwa#62
* The Sass Grunt task can be run with "grunt sass:dist" or the Sass source can be watched/built continuously with "grunt dev"
* The "grunt default" task now runs the Sass build task and the new karma unit test task.
* "npm test" now runs "grunt default" (build Sass -> run unit tests)
* Added a top level jshint task that can be run with "grunt lint"
* Added grunt-contrib-sass and grunt-contrib-watch to make the new grunt tasks possible.
* Also, moved the grunt-related packages to devDependencies in package.json