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.
Ha! Even better: the whole `NettySSLSupport` class doesn't even exist in 2.2.1. Also it appears that "NativePRNG" is no longer a supported option. I'm crafting an additional commit to point to the 2.2.1-specific reference configuration for akka remoting (an annotated config file).