Add HTMLBars `makeHandlebarsCompatibleHelper` function.

* `makeHandlebarsCompatibleHelper` will be setup at `Ember.Handlebars.registerHelper` in the future.
* HTMLBars compatible helpers from the container need a `_isHTMLBars` flag so that we do not wrap them.
* Make `lookupHelper` wrap any `helpers:` lookups from the container that do not have `_isHTMLBars` set
  in a new `makeHandlebarsCompatibleHelper` wrapped helper.