Document key, namespace, url, and transform customizations

This adds documentation for custom keys:

```js
App.Person = DS.Model.extend({
  lastName: DS.attr('string')
});
DS.RESTAdapter.map('App.Person', {
  lastName: { key: 'lastNameOfPerson' }
});
```

and namespaces:

```js
DS.RESTAdapter.reopen({
  namespace: 'api/1'
});
```

and url/hosts:

```js
DS.RESTAdapter.reopen({
  url: 'https://api.example.com'
});
```

and transforms:

```js
DS.RESTAdapter.registerTransform('coordinatePoint', {
  // ... you get the idea.
```

**also** a `require 'yaml'` line in the `Rakefile` for Ruby 2.0 compatibility.