Handlebars: support layout and partials for handlebars + sails-generate-views-handlebars

Hi.

I would like to propose support for layout and partial with Handlebars. The proposed PR contains little change to support a 'layouts/layout.handlebar' as the default layout and a 'partials/' folder for any partial you can use.

Also, I have a complete repository for sails-generate-views-handlebars (https://github.com/jmtexier/sails-generate-views-handlebars) that I would like to submit as a default generator for --template=handlebars (actually the PR will make most sense combined with this default generator)

This repo follow the same pattern and logic as the other sails-generate-views-* but is adapted for handlebars with default examples of i18n support, partials and default layout.

Thanks!!!
/Jim