Wrap Handlebars in a template adapter, Part II

This is an evolution of #19.  Unfortunately had to open a new PR because I couldn't push commits to @hurrymaplelad's branch (or my Git-fu isn't that strong).

I *think* this is all ready to go.  It uses [airbnb/rendr-handlebars](https://github.com/airbnb/rendr-handlebars) (forked from @hurrymaplelad).  I've simplified its API so that instead of:

```js
this.app.templateFinder.getTemplate(templateName);
this.app.layoutFinder.getTemplate(layoutName);
```
we have:
```js
this.app.templateAdapter.getTemplate(templateName);
this.app.templateAdapter.getLayout(layoutName);
```

Before we merge into `airbnb/rendr`, we need to figure out how to properly package the client-side files for an app, which I'm proof-of-concepting with `airbnb/rendr-app-template` (view the [template-adapter branch](https://github.com/airbnb/rendr-app-template/compare/template-adapter)).  It works fine server-side, however because we're using Stitch to package for the client, we'll have to do something like use Grunt+Browserify to compile a client-side `dist` file, because Stitch doesn't automatically package up the module graph.  This is another reason to switch to Browserify from Stitch, but that may be a lot more work.