Add controller extensions and Responder

* enables the use of `respond_with(@object)` to render Handlebars
templates on the server side.
* opt-in by require/include. It is not loaded by default into
handlebars_assets.
use Responder's reader methods...

…in order to be less susceptible to implementation changes.
Change supported_formats to :js , :jst

Sprockets renders show.html.hbs to show.html, and then refuses to
include the file because it’s text/html.

So we work around it, and require that the file is named
{action}.js.hbs or {action}.jst.hbs
Merge pull request #1 from wideopenspaces/adds_responder

Adds responder
Merge latest 0.17 changes and rearrange a bit

All server components are in the HandlebarsAssets::Server namespace and
are not loaded by default.

`require ‘handlebars_assets/server` will activate the server-side
rendering components.
Merge pull request #2 from wideopenspaces/tmp

Get server-side rendering working with the latest version