Add compress step via grunt-contrib-compress

This PR is based on this issue [#405](https://github.com/yeoman/grunt-usemin/issues/405).

Adds compress as the last step of the flow. Requires that you tack ```.gz``` on the end of the filename in the usemin block like so:
```html
<!-- build:js js/vendor.min.js.gz-->
```

Borrowed the majority of this from ```config/uglify.js```