Switch to Brunch; update Ember/Handlebars; use ES6 modules compiled to AMD

* Update Ember to 1.2.0
* Update Handlebars to 1.1.2
* Use ES6 module transpiler to compile to AMD-style modules
* Use module loader from ember-app-kit
* Removes Gruntfile and grunt option configs
* Adds Brunch config.js
* Update README with new commands
* Update TODO
* Temporarily removed sprite sheet creation
* Temporarily removed image optimization
* Static assets are now located in "app/assets"
* "public" is no longer a committed directory; Brunch puts compiled output here.