Added options for relationships

I added the possibility to declare omitted fields for relationships, e.g.:

```js
keystoneRest.exposeRoutes( keystone.list( 'Foo' ), {
  get : {
    omit          : ['password'],
    relationships : {
      bar : {
        omit : ['secretField']
      }
    }
  }
} );
```

It's still backwards compatible though, i.e. you can still do this:
```js
keystoneRest.exposeRoutes( keystone.list( 'Foo' ), {
  get : {
    omit          : ['password'],
    relationships : ['bar']
  }
} );
```