Handlebars Template Helpers

Now that Handlebars is the default view engine, it would be great to expose more functionality. Template helpers can simplify common patterns in templates. I added `HandleBarsAdapter.register` in `hb.server.js` and a test. Since this is only applicable to Handlebars, user must require 'mojito-hb' and register like so:

```
Y.namespace('mojito.addons.viewEngines.hb').register('helper', function (options) {
    return 'CustomHelper';
});
```

You would use it like so within your template:
```
{{helper}} or {{#helper}}{{/helper}}
```

For now, server side only. See http://handlebarsjs.com/block_helpers.html for more info.