Reimplemented `rootForType`

Currently if you have a double-barreled model name the AJAX request is to the camelized version of the URL.

```js
adapter.buildURL("lineItem") => "/lineItems"
```

This PR doesn't change the default but adds back the `rootForType` method for customizing the default.

An example is given for hooking it up with AMS.

I personally think the default implementation should probably be changed as well, although that isn't included in this PR