allow attributes to be excluded via the attrs hash (similar to readOnly: true in pre-beta)

refs #303

As per a discussion I had with @igort and @tomdale yesterday in #emberjs-dev:

adds a shorthnad versus overriding serializeAttribute or
serialize and using this._super

I'm also thinking it would be handy to allow some sort of array syntax for declaring multiple properties if you have a lot:

```javascript
App.PersonSerializer = DS.JSONSerializer.extend({
  excludedAttributes: [ 'admin' ]
});
```

Thoughts?