Make HTMLBars helpers instances of a Helper object.

As requested by @mixonic in https://github.com/emberjs/ember.js/pull/9631#issuecomment-63501759.
Actually we cannot do that as things stand right now.  The `helperFunction` is given a specific context (and not allowed to keep its own) via something like `helper.helperFunction.call(view, [params, hash, options, env]);`.  So if we move this to the shared prototype it will not be able to find the original `fn`. 

I'm open to discussing refactoring away from the view being the context of helpers, but it was specifically requested to be this way by Kris, Mat, and Martin so I think they have thought it through...