containerization of transforms

Extracted this from some [EPF](http://epf.io) work I am doing :). With this PR, transforms will now be looked up on the container. I.e. custom transforms can now be defined conventionally (no more `registerTransform`).

```
App.Post = DS.Model.extend({
  title: DS.attr('title')
});

App.TitleTransform = DS.Transform.extend({
  serialize: function(title) {
    return title.toUpperCase();
  },
  deserialize: function(serialized) {
    return title.toLowerCase();
  }
});
```