support storing and fetching links

Hi there, I've put together some support for storing and fetching riak links (not following links, yet!) This implementation introduces the RiakLinker[T], and RiakLink which puts that information into the RiakMeta[T] and RiakValue.

I've included a spec for the link parser and for basic store/retrieve.

```scala
  implicit object EntityRiakSupport extends RiakLinker[Entity] {
    def links(value: Entity) = Set(new RiakLink(bucketName, value.parent, "parent"))
  }
```