Add OffsetRepresentation

`PaginationRepresentation` is very handy, but is not reusable when the API use a offset/limit/count collection definition.

I've created `OffsetRepresentation` to fill this common use case. There are some similarities to `PaginationRepresentation` but subclassing/abstracting in this case seems overkill to me.

The name can be improved though... `OffsetLimitRepresentation`? `OffsetPaginationRepresentation`?