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
Nice catch! Let me update the patch in order to:

* Take into account the use of HTTPS for indicating extension relation types
* Take into account that multiple relation types may be indicated