Wrap Handlebars in a template adapter

Workin on #9.  Wrapped all references to Handlebars up in [rendr-handlebars](https://github.com/hurrymaplelad/rendr-handlebars) in preparation for the forthcoming rendr-teacup.  rendr-handlebars is configured by default.  Will write tests if the direction feels right.

Assumes templateAdapters are wrapped up in a module that can be required and exports a `templateFinder` and a `layoutFinder`.

Choose a templateAdapter by setting `templateAdapter` on the render `App`.  In the app-template, this could be configured on one line in any of `App.defaults`, `server.initApp`, or `renderApp.templateAdapter` in the config files.