Requests against relations

~~Required for easybib/issues#840~~

Tag ~~0.6.0~~ 0.5.11

This allows us to make new PUT and POST requests on relations directly against their "parent" Resources, e.g.

```php
$user = $apiTraverser->getUser();
$user->post('projects', $projectDataArray);
```

### ~~Breaking changes to resource/relation API~~

Retaining the deprecated methods, which now call the new methods