+http: Add basic Link header handling

This adds *partial* support for the Link header described in 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`, `anchor` and `title` params are (partially) implemented. My use case is that I want to support the GitHub pagination style (http://developer.github.com/v3/#pagination) using the Link header without having to resort to RawHeader. 

This pull request works for my use case, but there are link-params I haven't even attempted to implement yet. I am happy to expand the support for more link-params, but I need a bit of guidance of how to do that. I am currently stuck on how to construct rules for this grammar:

    relation-types = relation-type
                 | <"> relation-type *( 1*SP relation-type ) <">
    relation-type  = reg-rel-type | ext-rel-type
    reg-rel-type   = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
    ext-rel-type   = URI

Another problem I'm having is that Uri parsing is not provided as a rule in the ProtocolParameterRules trait. Additionally, as far as I can see there's no current way to say that "this matches only a fully qualified URI".