Add support for deeply nested handlebars templates

Hi,

I run into a minor issue.

In my yeoman app, if I put handlebars templates into a second depth level of `app/templates`, they are not compiled into `compiled-templates.js`.

I mean that in this hierarchy, `C.hbs` is not processed but `A.hbs` and `B.hbs` are.
```
|- app
  |- templates
    |- A.hbs
    |- B
      |- B.hbs
      |- C
        |- C.hbs
```

I had to edit my `Gruntfile.js` as in the commit attached to this pull request to make this work.

IMHO, it should be the default.

Sincerely,