Export the helpers in the public API for inheritance purpose

In order to overwrite and inherit the helpers like this:
```js
HandlebarsFormHelpers.register(Handlebars);
Handlebars.registerHelper('label', function (input, body, options) {
  body = doSomeThingLikeTraduction(body);
  HandlebarsFormHelpers.helpers.label.apply(this, [ input, body, options ]);
});
```
It would be fine to add project specific mechanism.

I do not know if the tests are carried out in the best way.