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.
My thinking was I didn't want it to fail in some situation where the value was "falsy", but not actually false.  I can change it to `==`.  I'm not so sure about not having any check (personal preference) but I can do that as well.  Just let me know.