Provide standard timestamps

Provide created_at and updated_at timestamps for resources by default, e.g:

{
  // ...
  "created_at": "2012-01-01T12:00:00Z",
  "updated_at": "2012-01-01T13:00:00Z",
  // ...
}

These timestamps may not make sense for some resources, in which case they can be omitted.