RESTAdapter learned side-deleting.

If the adapter has a deleteMapping property, records can be side deleted as
well as sideloaded when responses set this property to a hash of type names
to list of ids.  For example:

```javascript
  store.adapter.deleteMapping = '_deleted';

  // POST /people
 {
   person: { id: 1, name: "David" },
   groups: [{ id: 1, name: "Coffee Addicts" }],  //sideloading
   _deleted: {                                   // side deleting
     groups: [ 3, 4, 5 ],
     people: [ 6 ]
   }
 }
```