Add HALPresenter and ShafClient to Ruby Libraries

[sammyhenningsson]

I've written two Ruby gems to work with HAL, which I think should be added to the Library wiki.

• HALPresenter (a serializer)
• ShafClient (a client)

And perhaps also a framework called Shaf which is used for building REST APIs. Those APIs uses HALPresenter and thus also the HAL mediatype.

[sammyhenningsson]

I just saw (#27) that the preferred way is to edit the wiki myself.