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.
That link doesn't give much help -- not a lot of details there. I might suggest https://github.com/akka/akka/blob/v2.2.1/akka-remote/src/main/scala/akka/remote/transport/netty/NettySSLSupport.scala instead, since you can actually dig through and see what's going on.