Use Grunt to pre-compile Handlebars

The main stuff in here adds the necessary files to the Backbone reference implementation so that you can perform the precompilation of Handlebars templates by running `grunt handlebars`. It also namespaces those templates as `Ultracart.templates['template_name']`. Updated to Handlebars v2.0.0.

The second commit in here amends cart.html to use the latest version of Handlebars.js (minified) which is obtained using Bower. The  '.gitignore' file is excluding the `bower_components` and `node_modules` directories from the repository; might be easier to add those in.