[bz5800733] Handlebars Template Helpers (second attempt after #922)

This PR introduces the concept of helpers thru an addon called `mojito-helpers-addon`. Any mojito requiring this addon will be able to:

 * register a helper function at the instance level
 * register a helper function and expose it at the request level (when in the server)
 * register a helper function and expose it at the page level (when in the client)
 * expose an already registered helper function

## API

 * `ac.helpers.set(name, func)` to set a local helper
 * `ac.helpers.get(name)` in case you want to get a handler on a global or even local that was registered somewhere
 * `ac.helpers.expose(name, func)` to make it available locally but also exposing it globally
 * `ac.helpers.expose(name)` in case you want to expose an existing helper.

## How does it work?

Under the hood, any helper function, locally set using `ac.helpers.set()` or globally exposed by any of the parent mojits using `ac.helpers.expose()` will be passed into the handlebars render engine, so you can use them in your templates/views.

## Limitations and things to keep in mind

 * You will be able to override any default helper provided by Y.Handlebars since the custom helper functions will have priority. This also mean you can break the default handlebar helpers, so be careful.
 * You will not be able to use helpers when using `mojitProxy.render()` or `mojitProxy.refreshView()`, only thru `mojitProxy.invoke()`.