Server-side template rendering with Handlebars & HandlebarsAssets.

This is built on the work of @deepak, @zohararad, @AlexRiedler and @variousauthors in #46. Only the controller/responder code is my own. 

I'm putting this up mainly for feedback and improvement, and perhaps paving the way for inclusion in the main gem or development as an enhancement gem. 

As far as I am aware it only supports get requests on a single resource (i.e. 'show') at the moment; I have tested that. 

It adds support for the use of `respond_with(@object)` style rendering when paired with `respond_to :hbs` (or :handlebars), eliminating the need for either of the following cases as implemented in #46:

```ruby
respond_to do |format|
   format.html { render template: '...', locals: @object.to_hbs }
end
```

```ruby
respond_with(@object) do |format|
   format.html { render template: '...', locals: @object.to_hbs }
end
```

It does require a `:to_hbs` or `:to_handlebars` method on your resource, as it is used by ActionController::Responder to convert the resource.

It supports responding to multiple mime types (`respond_to :hbs, :json`) within the same controller and action, as well.

I don't know if it works with hamlbars or slimbars yet, but I'm interested in implementing that support, as I will likely need it in the near future.

No tests, but I'll see what I can do about adding tests for the code as soon as possible.

Finally, I tried to keep the server-side code as separate as possible. It is not loaded by default when the gem is included in a project, and needs to be required using `require 'handlebars_assets/server'` and included into a controller before it fully activates.
The only other concern which I thought of last night (but had already been on the way to bed) was that the context might not be thread safe.