Add GiftCard resource

This adds `GiftCard` resource which is basic CRUD and a `disable` method.

### Review

@csaunders
I'll give it a try. I found this too: https://github.com/rails/rails/issues/9121 but it seems like a fix was made to Rails 3.0 (https://github.com/rails/rails/issues/9121#issuecomment-12895652).