+http: Add basic Link header handling

This adds *basic* 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 implemented so far.

Fix failing spray-can-tests after adding link header to the HttpParser.