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.
What about putting here explicitly offset as an argument and set it to some default value?  It will increase readability and will be much easier to understand where to set it to false. Especial that you are mentioning about that argument above but it does not appear anywhere.