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 for explanation, but are you sure that it will work like that? Because:

    2.1.2 :008 > "falsy" === false # if it will not be a string any way we will get error here
     => false 

So any way it will pass. Much better protection will be just check if it is a Numeric like that

    if  options[:offset].is_a? Numeric
      ...

as everything except Number - should be trade as false, what do you think about that?