Added the ability to filter ActiveSerializer attributes

e.g. for attr1, :attr2, :attr3

```
{only: [:attr1, :attr2]} => :attr1, :attr2
{except: [:attr1, :attr2]} => :attr3
{only: [:attr1], except: [:attr2]} => :attr1
```

'only' takes precedence over 'except'

This enables you to pass arguments in from controllers using `respond_with @resource, only: [:id, :something]`