Fix external links in Scaladocs.

This addresses issue #41, by just taking the path of least
resistance and entering explicit URLs for the three
external references we currently have.
My initial thought was that these should link to the apidocs, since that's where we're coming from. That said, I do see what you mean about it not being much help. Since these are links in a comment field, I'd suggest we just include both links, as well as perhaps http://doc.akka.io/docs/akka/snapshot/scala/remoting.html#SSL. As long as they are well annotated, an interested person can easily follow the link they want.