feat(Response): Add a response helper for setting the Link header

This patch implements an "add_link" method for the Response class that
apps can use to add one or more Link header values to a response.

See also: https://tools.ietf.org/html/rfc5988

Closes #291
What of https?
Also reading the RFC, [section 5.5](https://tools.ietf.org/html/rfc5988#section-5.5) seems to indicate that `rel="start http://example.net/relation/other"` is OK, which would presumably need encoding.