Hack a prototype for asset fingerprinting together - not for merging, just for discussion

Meant to do this this week, actually for a while.

Just wanted to gather feedback on the approach first before making the final solution, which would be:- a new repository that provides a `gulp hash` task (a light wrapper around `gulp-hash`) and the Handlebars helper.

Final approach something like [psuedo code, obvs]:-

Each app would then have in its `server/app.js`

```js
// […]
var app = express({
    helpers: {
        // other helpers
        asset: require('next-asset-hash/handlebars')
    }
});
// […]
```

And in their gulp files:-

```js
// […]
var assetHash = require('next-asset-hash/gulp');
gulp.task('hash', assetHash);
// […]
```

And in their make files:-
```Makefile
build-production:
	gulp
	gulp hash
```