ModelSync.REST: Add entityLocator

Web frameworks like Rails, add a resource prefix be default to JSON returned.

This adds an option to ModelSync.REST to provide a `entityLocator` that will
automatically be added on `save()` and removed on `load()`

A prefixed response looks something like this:
```
GET /user/1.json

{
  "user": {
    "name": "Simon"
  }
}
```

@ericf There are several ways to achieve this, I went down a few paths and ended up on this, because it has no impact on current consumption of ModelSync.REST. 
I don't particularly like my `_addEntityLocator` method - Would love suggestions.