Offset options, description, more specs.

This PR includes a couple of changes around the `offset` option.

- Added a default description.
- Made it possible to pass a default offset value similar to per_page and max_value.
- Added feature so you can pass `offset: false` and the offset parameter will not appear in the API.
- Added specs to test these assumptions and additional specs to test setting headers and basic pagination implementation.
Thanks, will be great if you could remove them (keep in mind that there are in many places also below) as this is unnecessary noise in the code.