add Link header containing next and previous links

To make it easier for consumers of atom feeds to paginate, this change will add the previous and next links in the HTTP response header. This will be the Link header, following this RFC:
* http://tools.ietf.org/html/rfc5988

A Link header example looks something like this:

Link: <https://localhost/myown/feed/?marker=urn:uuid:560490c6-6c63-11e1-adfe-27851d5aed13&limit=25&search=&direction=forward>; rel="previous", <https://localhost/myown/feed/?marker=urn:uuid:560490c6-6c63-11e1-adfe-27851d5ff111&limit=25&search=&direction=backward>; rel="next"