{{gh-path}} Handlebars helper, returning paths

closes #3718
- Uses utils/ghost-paths to resolve paths
- Usage: Assume 'http://www.myghostblog.org/myblog/'
- {{gh-path}} or {{gh-path ‘blog’}} for Ghost’s root (/myblog/)
- {{gh-path ‘admin’}} for Ghost’s admin root (/myblog/ghost/)
- {{gh-path ‘api’}} for Ghost’s api root (/myblog/ghost/api/v0.1/)
- {{gh-path 'admin' '/assets/hi.png'}} for resolved url (/myblog/ghost/assets/hi.png)