+http: Add basic Link header handling

This adds *partial* support for the Link header for RFC5988, Section 5. (See http://tools.ietf.org/html/rfc5988#section-5 .) 

Multiple link-values are supported, but of the link-params only the `rel` and `title` params are (partially) implemented. I wanted to be able to support the GitHub pagination style http://developer.github.com/v3/#pagination without having to resort to RawHeader.

I am happy to expand the support for more link-params, but don't want to spend time on that if there's no appetite for it.