use GraphModel in graphModel.js

@haosharon per your question about not seeing the `GraphModel` class definition, I've changed the code to be more appropriately named, but **before this change it was still "correct."** Here's why.

`requirejs`'s `define` function translates the strings in the array (which is it's first argument) by looking up the corresponding *file name* and seeing what the `define` function in that module *returns* - it does **not** look for variable names within that module.

So, I could also have returned an anonymous class definition in `graphModel.js` like this.

```javascript
// alternative graphModel.js implementation
define(["underscore", "backbone"], function(_, Backbone) {
return Backbone.Model.extend({
// implementation here;
});
```

^that is honestly better than it was before, but the changes made for this pull request are probably the best.

**Is anything about that unclear?**