add 'time-grunt' and improve the Gruntfile.js a bit

The Gruntfile now shows time graphs after a build has been done. It also uses 'load-grunt-tasks' for loading grunt tasks. For reference, these two modules are also used in the official [yeoman/generator-webapp](https://github.com/yeoman/generator-webapp).

I also added 'use strict' to enable strict mode. This prohibits the use of the variable 'package' so I inlined that bit.