Add support for GruntJS, with JSHint

This is like my [other PR](https://github.com/imsky/holder/pull/42) but this also includes JSHint. To run JSHint just type `grunt` or `grunt jshint`.

I disabled several rules so your current code would pass, but I had to make some minor changes (like == to ===) to make it pass. There is one error left though, and I left it for you as it makes sense that you fix it unless there's a good reason to keep it as it is:

    [L40:C35] W103: The '__proto__' property is deprecated.
    var proto = this.__proto__ || this.constructor.prototype;