Add headers to response

[mehmetuygun]

In order to check the following attributes X-Limit-Per-Second, X-Remaining-This-Second, X-Limit-Per-Day, X-Remaining-Today, we need to be able to get headers.

[rhysnhall]

Apologies on the radio silence. This repo is now being maintained again. A major update for the package has been released that will impact this PR.
I do approve of this idea though, and if you can revise for the updated version I will be happy to merge. However, the headers need to be returned recursively either through the returned Resource or Collection, or a static property on the Etsy or Client class (ideal).