Add Backbone uniphil/js-tools

There are amd-compatible versions of underscore and backbone available, which gulp will find by using the -amd suffix, as in bower.json:

  "dependencies": {
    "underscore-amd": null,
    "backbone-amd": null

That is the main notable thing about adding backbone. It is given a name in [app/scripts/main.js] just like jquery so that it can easily be required.

The structure of these changes is inspired by the same tuts+ tutorial as the last.