Add `difference` underscore.js method to Backbone.Collection

Adds the `difference` underscore.js method to Collection, allowing for this

```
var arr = _.difference(myCollection.models, excludeThese)
```

to become this

```
var arr = myCollection.difference(excludeThese)
```

I added a test for it as well, which passes. However, when I forked off master, a failing test already existed.