allow passing list of fields for validation to save

when calling `save`, it could be helpful to be able to pass the list of fields to pass to `validate`.  for example, a schema might define an `id` property that's required but when creating a new object the `id` is assigned by the server.  in that case, being able to validate all the other fields (rather than completely skipping validation with `options.skipValidation`) would be useful.

```js
store.create(data).save({ validateFields: [ 'foo', 'bar' ] });
```

the PR will be trivial - i'll try to get to it soon.