add example usage

- Handlebars.registerHelper
- Handlebars.registerViewHelper
- update test to DI thorax, which is required for
  Handlebars.registerViewHelper to work

The notes include references to examples of how to test the helpers
which are not included in this PR but will be forthcoming in others.

If these references should be removed, let me know and I’ll do so

/cc @eastridge, @ryan-roemer