Use `rendr-handlebars` template adapter.

These are the updates needed to fit with the upcoming merge of airbnb/rendr#56.  The only real changes are to `Gruntfile.js` for packaging, and `app/app.js` to show how to register Handlebars helpers.

I think when we merge in airbnb/rendr#56, we'll bump the version to `0.4.8-alpha.1`, and so I'll update the `rendr` ref in `package.json` in this branch to reflect that.  Exciting!!!

/cc @hurrymaplelad