[Fixes yahoo/mojito#1127] Allow `Y.Handlebars.registerHelper` to register helpers

As per http://yuilibrary.com/yui/docs/handlebars/#defining-custom-helpers, the Handlebars helpers are defined using `Y.Handlebars.registerHelper`.

However, any helper registered using the above way are not available while generating views from hb templates on server.

Use case: Adding set of global handlebar helpers that works on both client and server. (helpers like pluralize, lowercase, uppercase, truncate etc. which are independent of mojit). More like, I want the Handlebars engine to be enhanced where I've access to a bigger set of helpers with almost no extra effort.

I understand `ac.helpers.set` and `ac.helpers.expose` are available, but I find a couple of concerns using this approach:
1. Needs access to `ac` which is available in each function of controller, and hence every function needs to attach these helpers.
2. Incompatibility with existing YUI code, which declared helpers this way.