Added hypermedia appenders

This pull requests is a revised version of: https://github.com/JakeGinnivan/WebApi.Hal/pull/57
I have also added a wiki page to describe usage: https://github.com/JakeGinnivan/WebApi.Hal/wiki/Hypermedia-configuration,-container-and-appenders
I am changing this back to `return new Link(newRel, CreateUri(parameters).ToString(), Title);` due to a merge conflict, was not sure the reason for the change so went with the original.

Let me know the reason, we can change in another PR