add requirejs

This commit roughly follows the [getting started guide from RequireJS](http://www.requirejs.org/docs/start.html).

A single script tag in [index.html](app/index.html) loads requirejs and points it to the application entry-point, [app.js](app/app.js).

[app.js](app/app.js) handles configuration for requirejs, and then simply requires the [main module](app/main.js).

The `paths` config is used to specify the actual path to the jquery module, allowing other modules to simply `require("jquery")` by name.

[main.js](app/main.js), finally, is the entry-point for application code. Later it might initialize a backbone `View` or something; for now it just `require`s jquery and a trivial test module, and calls a function from the module.