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.
Is there any reason why we have to use here `===` instead of `==`? 
If the offset can be set to any number or false why not just use:

    unless options[:offset]

    end