Refactor build and test tasks

* use [grunt-es6-module-transpiler](https://github.com/joefiorini/grunt-es6-module-transpiler)
* use [load-grunt-config](https://github.com/firstandthird/load-grunt-config) and [load-grunt-tasks](https://github.com/sindresorhus/load-grunt-tasks)
* move tasks configurations to `tasks/options`
* enable ECMAScript 6 specific syntax on `.jshintrc`